I've build levels for Quake, Doom, Unreal and Undying as well as Half Life and Deus Ex, so a lo of Unreal engine plus some Id engines and Source.
I've also built some dungeons for Oblivion but never fully finished them - I realised I just didn't have time to spare for making stuff anymore.
Best accomplishment, coming second place in a level building competition and getting some nice comments from the developers.
I'd love to have the time to actually build more but I've had to let it go. I did download the latest Unreal SDK to play with a bit, but haven't really done much more than that. BTW you can use it to build anything that is non-commercial I believe (I never really read the terms fully but just skimmed it).
Try to be reasonable... its easier than you think...
