You could use anything to do it. if you want free Mozilla has a decent composer built in as long as you use it in code view also you can get nvu which is mozillas composer as a standalone application http://www.nvu.com I think. Alternately you could use Topstyle (Which costs money but kicks ass) or if you are on a Mac BBedit (Around $80 or so same as topstyle), skEdit, and plenty of freebies too such as Taco etc.. Best bet would be to check www.versiontracker.com and/or www.sourceforge.net for free web design solutions. If you can run xWindows etc CSSed is one hell of a powerful CSS Editor and there are also plenty of free open source projects for HTML etc.

Personally I use skEdit http://www.skti.org to hand code all my XHTML Strict and CSS. When I run windows on my MacBook Pro, I use notepad.