Unity maybe?
Not free last time I checked.
what does dim3 lack for making these outdoor worlds, Oddity?
Dim3 plays nicely with closed hallways and the like; the sort of thing you see in the early Countdown screens. Open environments slow it down, because everything is in view all the time. You should take what Oddity says about performance with a tiny grain of salt though, since he works on a dinosaur of a computer. 
That's not true, but you have to understand how dim3 optimized.
If you have fog on, any mesh that outside the fog outer radius is eliminated.
If you have any breaks (like ridges, mountains) use areas to do eliminations.
Anything outside the view port is also eliminated.
If everything is one big mesh, though, that'll be a problem.
If somebody has a project that's extra slow and a big outdoor area, send it in to me and I'll see what I can do to optimize it.
[>] Brian