I am doing some work right now with procedural generation of game contents (placing things on levels/extending levels in intelligent ways). I don't want to re-invent the wheel so i want to export my output into an existing level format. Any suggestions? here are my requirements:

1) can get the file format description online somewhere
2) is good for both indoor and outdoor (landscape) levels and mixes of the two.
3) Is at least marginally supported by some open source or free 3d engines so i can view the output of my works
4) can support placement of items, entities, and scripted events.

I am leaning towards the quake 3 level format since it is well documented online. Any other suggestions that might be more appropriate?

well, from a level standpoint, quake 3 levels are awesome because the format has a nice editor, performs lightmapping, and compiles down to bsp trees. of course, if you're going to be altering the levels in-game, bsp trees and lightmapping probably aren't going to work.

so i'd have to recommend the directx mesh because it's well-documented and supported by lots of editors. and they have all sorts of cool operations, like vertex optimizations and progressive mesh functions, which would help in a dynamic level.

I found this one being well documented and free.
http://www.zalsoft.com/articles/Getic%20BSP%20File.html

Here is the editor:
http://www.zalsoft.com
And here the file format:
http://www.zalsoft.com/articles/Getic%20BSP%20File.html

if you want support from many content-generation-tools, you might want to look at COLLADA or FBX...