New World Editor

http://www.3d1.org/

It has affiliations with the 3DState engine (probably written in it too). Don't know if it's any good, but it has a nice feature list. I couldn't find CSG on the list but I did see 'cutting' which might be the same thing.
In the e-mail I got sent it said that its 'free for non-commercial use', but didn't mention how much it costs otherwise. Have a look anyway... ;)

Hi skitchy

you need to regester for a seriel number(its free)

dave

"Commercial Use is Illegal." is clearly visible on one of the screenshots. Which is kinda weird. You'd think they'd just charge for commercial use.

@sybixus

sorry i meant the seriel number

"Commercial Use is Illegal."

dave

I know what you meant. I wasn't correcting you. Just noting it for anyone who was interested.