Old thread, but for completeness there is also Anylogic and pyabm which can be used for ABMs.
I have experience programming agent-based models in several environments / languages. My opinion is that if you want to implement a relatively simple model, use Netlogo. It's also possible to use Netlogo for heavy-duty models as well (I've done this successfully), but at some point the flexibility of a programming language like java/python/c++ outweighs the convenience of the native methods available in Netlogo, especially when performance becomes a major issue.
Repast is becoming a bit bloated. If you are an experienced programmer, all you really need to start building an ABM is the ability to schedule events and draw random numbers. The rest (defining agents / environments and their behaviors) you can craft on your own. When it comes to managing the objects in your model, use the regular data structures you're used to (arrays / hashes / trees / etc.). To this end, I'm developing a very lightweight Java library called "ABMUtils" (on github) that implements a scheduler and wraps a random number generator. This is in the early development stage but I expect to flesh things out (keeping it simple) over the coming months.
