I always recommend downloading Skitch
Nice tool for easy uploading of images etc.
In relation to this, if you want a photo editor that is powerful and doesn't rely on X11 (Gimp does on the Mac and it's pretty bad), take a look at Seashore. It seems to be the only decent free alternative to Gimp that works well. Hasn't been updated in ages but support doesn't seem to have been totally dropped.
As I always suggest to new Mac users, do try out the standard applications first. Safari is probably the best browser out there on the Mac at the moment (Firefox is Firefox, Chrome isn't quite as stable for a lot of users and GPU acceleration is ****-poor at the moment, Opera is a matter of preference), and Mail is simple and powerful. iChat is also fantastic and simplistic, and Messages (iChat's successor that's available as a beta right now) is also great, though buggy in comparison.
Other recommendations are The Unarchiver (free, integrates well with Mac OS X and supports all the file compression formats you'll ever need), Perian, Fluke (for FLAC) and Flip4Mac (WMV) for your codec needs, Gruml (Google Reader client that is free), Mindnode (mind-mapping), Wunderlist (task list), Transmission (BT client), Cyberduck (FTP, SSH etc), Carbon Copy Cloner (drive cloning for backups, if not use Time Machine as it works well).
Those are some that I use, and they're useful. Pages is also good, provided that you don't need brilliant Word compatibility, though that's another story.
Finally, Office for Mac if you need Office, don't skimp and go for LibreOffice and the like, as they're awful.
