A bit late but, hey...
I´ve used Opera´s browser as long as I can remember on my Pc´s and thought that it´s as good as it gets, but no. Opera is even better on a Mac. Get it, try it, love it.
Onyx - Cleans up your Mac, IF it starts getting slowish, like. (Careful, it´s a powerful tool. If you don´t know what happens, don´t do it!)
OpenOffice.org - For all your Office needs.
Gimp - For great picture editing.
VLC - Plays 99% of the media out there. Good movie player.
HandBrake - Encoding media files from "whichever" to "whatever".
FlashVideoDownloader - Browser-widget that allows you to download a bunch of videos that you otherwise can´t.
I use 9 separate widget-clocks simultaneously, in a row, on the Dashboard, as a "world clock" with local time in the middle. Also, 1 calendar, 1 translator, 1 multi-converter, 1 dictionary, 1 Wikipedia, 1 calculator and 3 weather-stations (1 for each city I frequently visit). The point is: Dashboard is really cool and useful. You can alter how you access Dashboard, Spaces, the Dock and other built-in apps in System Preferences.
Mac is, as you probably have figured out by now, full of cool widgets, gadgets and apps and don´t really NEED anything extra.
I converted from 15 years of cursing at the virus infected, blue-screened, noisy, outdated-within-6-months Pc´s to Mac, 2 years ago and never looked back.
Enjoy!