Well it depends on what your usual tasks are. But in terms of general-use applications...
* Adium -- Agreed with this choice; it's the best text messaging app on the Mac (it doesn't do video). Free and open source.
* Little Snitch -- A two-way firewall, letting you know when applications try to contact servers without your knowledge.
* Grab -- A basic screenshot utility which can capture windows, screens, or specific areas. This actually comes with the Mac, but because it's in the /Applications/Utilities directory a lot of people don't know about it. Snapz Pro is commercial software that can do full desktop movie captures.
* Speed Download -- A download manager, commercial software. Great for speeding up http and ftp transfers. Some people like using Firefox extensions for this or whatever, but I prefer a native app. YMMV.
* Max -- Audio file encoder. Free. Good for batch changing FLAC files to Apple Lossless format, and things of that nature.
* SuperDuper -- Clones a drive, making fully bootable backups. Flexible options. Does what it does very well. Commercial software.
* The Omni suite of software is great for various business needs. OmniGraffle is great for flowcharts, org charts, that sort of thing. OmniPlan is a nice competitor to MS Project.