also check out
www.macupdate.com. It lists updates for many apps on a daily basis, and groups the more popular apps together at the top at the end of the day (links are always in red instead of blue). I've found this pretty useful when looking for updates/new apps.
The first things I always install, be it a fresh install, or a new mac are:
Perian - media codec for Quicktime. I prefer Quicktime over VLC, although I do use VLC for movie playlists. Perian just allows you to watch many, if not all, video files watchable with VLC, with Quicktime
VLC - media player alternative to Quicktime. Has many codecs built in, so no need for Perian.
Ejector - Ejector adds a little eject button in your menu bar, letting you eject disc images, ipods, cds, usb drives, etc... . I like to keep my desktop empty, so I use the Ejector dropdown menu to pick which image/drive to eject. Very useful.
Google Notifier - If you use a gmail account, this is pretty useful. Adds a menubar counter which displays unread emails in your gmail inbox.
OnyX - Like others have said, OnyX is for running maintenance scripts, and small customizations to your OS X. Running maintenance scripts is not really required, since OS X is pretty efficient when it comes keeping your apps running fresh, but hey, when in Rome right?
Inquisitor or
Glims - I used Inquisitor a while ago, adding a "search as you type" function to Safari's Google search field. Glims adds this feature, as well as other features, such as full screen browsing, keyword searching from your address bar, etc.. .
Also, let us know what you intend to use your mac for, as it will help us suggest more specific apps for you.
