Here is a list of the tools I use on my Macs.

For mail I use Gmail and it's web interface.
AOL/AIM and Google Talk instant message clients are both supported in Gmail

Adium for chat when AOL/AIM & Google are not enough. It seems to support just about everybody.

NeoOffice for Word/Powerpoint/Excel compatible 'office' stuff when I'm not using Google Documents

Nvu for HTML editing

Firefox for browsing with Foxmarks for bookmarks that are shared between your computers.

Seashore for basic image manipulation

Sketch-up from Google for sketches

Skype for free computer to computer voice calls

I also use Omni's OmniGraffle for making cool presentation graphics an charts. (not free but reasonable)

Built in to your Mac you have Time Machine for backups (you need to have an extra drive connected)
iPhoto for photo editing and management
iTunes and bunches of other cool things