Right, where to start?
You'll find there are arguments for either iWork or Office for Mac... I'm an iWork kinda guy and there is the benefit of being able to sync Pages, Numbers and Keynote across all your iDevices (as well as saving in .doc, .xls or .pst formats).
For photos and video you already have iPhoto and iMovie but when the time comes, Apeture and Final Cut are excellent.
Get yourself AppFresh to keep apps up to date, CleanMyMac for system maintenance. Calibre for books, Cyberduck or ClassicFTP for FTP, Gimp for graphics, Micro Video Converter to convert video for your devices, Stuffit or The Unarchiver for zip or rar files, Speed Download to manage downloads, Unison for newsgroups, Transmission for torrents, 1Password remembers passes and logins, Flip4Mac and Perian for wmv playback in QuickTime... There are some really cool ones like DaisyDisk, Delicious Library, CandyBar, Fontcase, Pixelmator and on and on and on... It can get a little expensive if you load up on everything at once. Get what you need now and add as you go.
Have fun with that Mac now ya hear.
