What's a good IDE for CSS that makes validation of the code as you type?
I would suggest Aptana or Visual Web Developer Express 2010
Aptana is Eclipse-based, cross-platform, needs no installation, just JRE. Pretty neat, I use it. In CSS it even provides code completion for selectors (based on what you have in HTML). If a line is invalid an error sign appears next to it, hover your cursor and read what's wrong.
Visual Web Developer is harder for me because I spent too few hours CSSing in it. From what I've seen it does a pretty decent job, but I still prefer Aptana.
Netbeans also has that and is free. And for what its worth there is also dreamweaver but I have no idea how good it is compared to more "serious" ide's.
If you use a Mac for any development then I would advise CSS Edit although I expect that is not the case.
All of the jetbrains IDE products (with the exception of the free CE edition) include a every good CSS editor.