X-Lite - SIP client. I tried some open source soft phones and couldn't find one that actually worked. X-Lite sure looks silly, but it works. I use it with WorldDialPoint.
Isolator - thing that obscures everything other than the foreground application. Good if you get distracted easily like I do!
Mozy - 2GB of free online backup. Referral link means that you get more than 2GB.
firefox-mac-pdf - a Firefox plugin that lets you read PDFs in the browser window, which is the one thing I really liked about Safari.