Unreal already has released it's development kit for free now Valve have released their one http://source.valvesoftware.com/sourcesdk.php for free - any modders here using other kits?

Are you looking for big name modding tools or the countless other smaller games that have released modding tools?

Actually I wouldn't mind knowing what other kits are used - I know some here used to mod for NWN and there are the 2 engines being used for Morrowind which names escape me now but don't really know of anyone using anything else big or small or what other games are being modded by the community

I use Unity for mobile dev but not modding. I wanted to use UDK 3 but I gave it a go and the workflow was too awkward, time-consuming and my PC couldn't run it that fast. If I were doing a top tier game I'd definitely use UDK but not for simple, rapid mobile games. Unity is expensive but I've found it to be so strikingly similar to the CS that there was a hyper-minimal learning curve.

If you're just talking about for MW then I use Matlab for programming the complex scripts first and then porting to MWSE. If I structure the code to only use things available MW( if, while etc) then it is much faster and I can get nice graphs to see where I'm going wrong .