TSim
I have been using Tsim for several months, and I think it has been updated since the last review because it now has Limit and Stop orders including trailing Stops. It also has the onscreen p/l that the previous reviewer mentions, it updates in real time which is great when the number is green but a bit depressing when red. It would be nice to be able to hide this though because it can be off-putting when in a trade. I dont use it as a simulator much so my comments mainly relate to real-money trading, but the simulator mode looks to be identical anyway.
I tried some of the other TWS APIs but I trade mainly stocks and they all seem geared up for futures. This one does stocks properly, and has no trouble with having different positions open at the same time which is what I need. I also like the eSignal linking. It is quite big and might take up too much room if you only have one monitor or a low resolution.
I cannot properly comment on the support because I have not really needed it it is pretty easy to use and the instructions are all on the website. I did send them one question by email and they replied the same day which is always a good sign.
In summary, it does everything I need, is miles easier to use than TWS, and has not broken on me yet so I am happy with it especially considering that it is free which is my favourite price.