- Thunderbird and Firefox with the del.icio.us plugin. I've also been using TwitterFox but DH has not been converted to Twitter just yet. Greasemonkey plugins are good too.
- BusySync which I am now using to synchronize iCal with Google Calendar.
- Flip4Mac which allows Windows Media files to play on your Mac via QuickTime
- Lingon, basically a cron job scheduler. A GUI to launchd to be a bit more technical.
- AdiumX, an instant messaging client that integrates most IM protocols (yes, even Lotus Sametime).
- Parallels or VMWare Fusion, both allow you to run Windows on your Mac if you need to.
- MissingSync if you still love your Palm
What Macintosh programs do you use?