I'll second TextWrangler as a decent text editor.

For CSS, the MacBreak Weekly guys like CSSEdit (http://macrabbit.com/cssedit/)

The biggest thing to do during the switch is not to find how to do your same work the same way as you did on Windows. You instead need to find out how to do your work the Mac way. As others have pointed out, window management is very different between OS X and Windows. Expose is a handy way to do it, with newer macs shipping with this defaulted to F3. Thats only one expose mode, play around in system preferences to see all three and assign hot keys. You can also hide the front most app with Command-H, or hide all others with Command-Option-H. I use an app at work called Spirited Away (http://drikin.com/spiritedaway/) that auto hides apps I haven't used in the past 2 minutes. On top of all that, Leopard has spaces to allow a virtual desktop environment to be used on the Mac.