Heller's Disease 
Normal development to age 3 or 4, then abrupt onset of fretfulness, negativism and anxiety.  Regression of mental development and gradual loss of speech.

 
LKS "Landau-Kleffner Syndrome" 
Also Acquired Childhood Epileptic Aphasia. Very rare disorder. Typical profile: normal development and age appropriate language first 3-7 years; Loss of receptive language while     retaining some expressive language; "Telegraphic" speech--few verbs; Suspicion of deafness;  Child frustrated, puzzled by change in himself; Autistic-like behaviors; normal or above 
normal non-verbal IQ score; abnormal EEG, with or without seizures. Some practitioners suspect that some cases of Childhood Disintegrative Disorder or "late outset" autism may be  Landau-Kleffner Syndrome.

 
Rett's Syndrome 
a neurological disorder that occurs only in girls. Unlike autism, girls initially show normal development, then revert. The initial symptoms include some that are associated with autism. 
    

From DSM IV: 
DIAGNOSTIC CRITERIA FOR RETT'S DISORDER   (females only) 

A. All of the following: 

(1) apparently normal prenatal and postnatal development

(2)  apparently normal psychomotor develop (3) normal head circumference at birth 

B. Onset of all of the following after a period of normal          development: 

(1) deceleration of head growth between ages 5 and 48 months

(2) loss of previously acquired purposeful hand skills between ages 5 and 30 months with subsequent development of stereotyped hand movements (e.g.  hand wringing or hand washing)

(3) loss of social engagment early in the course (although social interaction often develops later)

(4) appearance of poorly coordinated gait or trunk movements

(5) severely impaired expressive and receptive language 
development with severe pyschomotor retardation. 
 

Childhood Disintegrative Disorder  
Much rarer than autism. I've seen references which treat this as a synonym for Heller's Disease (see above).

From DSM IV: 

DIAGNOSTIC CRITERIA FOR CHILDHOOD DISINTEGRATIVE DISORDER 

A. Apparently normal development for at least the first 2 years 
after birth as manifested by the presence of age appropriate verbal and nonverbal communication, social relationships, play and adaptive behavior. 

B. Clinically significant loss of previously acquired skills 
(before age 10 years in at least two of the following areas: 

         (1) expressive or receptive language (2) social skills or adaptive behavior (3) bowel or bladder control (4) play (5) motor skills 

         C. Abnormalities of functioning in at least two of the following areas: 

         (1) qualitative impairment in social interaction (e.g., impairment in nonverbal behaviors, failure to develop peer relationships, lack of social or emotional reciprocity) 

         2) qualitative impairments in communication (e.g., delay or lack of the development of spoken language, inability to initiate or sustain a conversation, stereotyped and repetitive use of language, lack of verbal make-believe play) 

         (3) Restricted repetitive & stereotyped patterns of behavior,  interests and activities, including motor stereotypes and mannerisms. 

         D. The disturbance is not better accounted for by another specific Pervasive Developmental Disorder or by Schizophrenia. 
 

Tourette's Syndrome 
A condition thought to be genetic that causes uncontrollable motor and/or vocal tics. 

Obsessive-compulsive disorder  
OCD is a neurological disorder, cause uncertain. Clinical OCD has easily categorized symptomology that tend to occur at certain stages of life; counting and sorting and "evening 
out" usually start during childhood, "grooming" compulsions usually start at puberty, and "ruminating" (obsessions) usually begin during adulthood. 

Cocktail party speech syndrome  
A syndrome comprising the following characteristics:

(1) A perseveration of response, either echoing the examiner, or repetition of an earlier statement made by the child. 
(2) An excessive use of social phrases in conversation. 
(3) An over-familiarity in manner, unusual for one's age. 
(4) A habit of introducing personal experience into the conversation in irrelevant and inappropriate contexts. 
(5) Fluent and normally well articulated speech. 

Asperger's Syndrome  
Similar to Autism (most think of it as a high functioning autism) except that language development is normal or near normal. 
     From DSM IV (p77): 
 

         DIAGNOSTIC CRITERIA FOR ASPERGER'S DISORDER 

         A. Qualitative impairment in social interaction, as manifested by at least two of the following: 

         (1) marked impairments in the use of multiple nonverbal behaviors  such as eye-to-eye gaze, facial expression, body postures, and  gestures to regulate social interaction 

         (2) failure to develop peer relationships appropriate to 
         developmental level 

         (3) a lack of spontaneous seeking to share enjoyment, interests, or  achievements with other people (e.g. by a lack of showing,  bringing, or pointing out objects of interest to other people) 

         (4) lack of social or emotional reciprocity 

         B. Restricted repetitive and stereotyped patterns of behavior,  interests, and activities, as manifested by at least one of the following: 

         (1) encompassing preoccupation with one or more stereotyped and restricted patterns of interest that is abnormal either in intensity or focus 

         (2) apparently inflexible adherence to specific, nonfunctional routines or rituals 

         (3) stereotyped and repetitive motor mannerisms (e.g., hand or finger flapping or twisting, or complex whole-body movements) 

         (4) persistent preoccupation with parts of objects 

         C. The disturbance causes clinically significant impairments in social, occupational, or other important areas of functioning 

         D. There is no clinically significant general delay in language (e.g., single words used by age 2 years, communicative phrases used by age 3 years) 

         E. There is no clinically significant delay in cognitive 
         development or in the development of age-appropriate self-help skills, adaptive behavior (other than social interaction), and curiosity about the environment in childhood 

         F. Criteria are not met for another specific Pervasive 
         Developmental Disorder of Schizophrenia 
 

Semantic-pragmatic speech disorder (Semantic Pragmatic Disorder or SPD)  A communication problem with mild autistic symptoms and problems generalizing. 

ADD and ADHD "Attention Deficit Hyperactivity Disorder"  
Thought to be caused by a chemical imbalance in the brain, which results in a biological deficiency in a childs ability to concentrate. Diagnosis of ADHD is a grey area: there are 
18 criteria involved in identifying ADHD including such traits as inability to concentrate and aggressiveness. 

Manic Depression  
Autistic Children who have no apparent neurological basis for their disorders may actually be suffering from an inherited, early-onset form of manic-depression.

Klinefelter Syndromes 
Having an XXY chromosome. It is easily testable through genetic testing and occurs in about 1 in 1000 births. Often includes developmental and language impairment, and has been correlated with some kinds of withdrawn behavior. 

DAS "Developmental Apraxia of Speech" 
Also known as "Developmental Verbal Dyspraxia". A neurologically-based speech disorder observed in children learning to speak. It affects the rate of speech development, the number of sounds in a child's repertoire, and the child's ability to combine sounds during the production of words. 

Lactic Acidosis  
Ttendency to accumulate of lacitic acid in the blood. It's connection with autism is that it has been found to coincide with autism more than chance would dictate.

Prosopagnosia 
The inability to recognize faces, also known as face blindness.

Carnitine Deficiency  
A condition which can have symptoms similar to Reye's Syndrome. It can be caused by  genetic factors. 

Early Origins of Autism Ask an Expert on Autism Health Finder Talk to Autism Expert National Institutes of Mental Health Combined Health Information Database Abstracts on Autism