Question: I found out that render targets in c# / xna are not automatically disposed of, and you have to call the .dispose() member to get rid of them.

I thought garbage collection was suppose to automatically get rid of things after all their references are gone, what gives?

Is there anything else that is not disposed of automatically?

Answer: 
Even with automatic garbage collecting it is still a good idea to call `Dispose` when you are sure you won't use the instance again. 

Or use the `using` statement.

That way you can choose when you want to have the object "disposed" instead of waiting until the garbage collector does it.
My opinion is that if an object has a Dispose method, it probably needs to be disposed. I usually try to put it in a block with a 'using' statement. This will automatically call the Dispose method at the end of the block.