The first question is,
What work did you do with your Windows box? Games? Try the games channel on Apple's Quicktime site or insidemacgames.com for some ideas there. MS Office is available (and cheaply, too!) if you did general WP/Spreadsheet stuff, if you did PowerPoints, try Keynote instead. MUCH better. In general, though, for free/cheap stuff, I find I use iLife/Mail/Safari the most, along with Konfabulator (which is a must for any Mac-user, IMO).