An external disk for Time Machine backups.
I also use Carbon Copy Cloner to create a bootable backup on another external drive. I'm a backup freak; it's a belt and braces approach. CCC is nagware so see if you like it before buying.
Back-in-Time 2 - a better interface for restoring Time Machine backups. Demo mode is "fully functional" with the restriction that you can only restore the latest version of a file. Its initial value to me was the display which shows how much is backed up and when, so that I could work out what TM is doing.
Graphic Converter. Nagware (30 second delay on startup) which doesn't expire. You need a licenced copy for batch conversions.
NotLight. Free .An alternative interface for Spotlight. The early Spotlight interface was crap, so this was indispensable in earlier versions of OS X, but I still find it handy.
Firefox / Opera / other browser of choice.
Don't think much of iLife myself apart from iPhoto, but it's worth it for that.
OpenOffice for spreadsheets and formats that iWork or Office can't handle. The iWork spreadsheet Numbers is fine for noddy stuff, good for graphics, but compared with Office or OpenOffice it's too basic for professional use. iWork's Pages and Keynote are very good.
Disk Inventory X. Free. A graphical display plus directory tree of where your disk space went to. It churns for ages when you give it a disk to go at, but you are rewarded with a pretty and useful display. Just found out where I can free up 20 GB
If you use newsgroups, MT-NewsWatcher. Donationware.
Lingon. Explore your plist files.
Audacity. Free. If you want a more professional music editor, Amadeus Pro is well worth the money.
Soundflower. Free - redirect audio from one app to another. This was more useful on my old iBook which didn't have audio in, but I still occasionally use it to grab internet radio to disk.
Various utilities I grabbed from Apple's development site. There is some useful stuff there, often including sources, depending on what you want to do.
Avoid macports or fink. If you need something from unixland which doesn't have a native OS X version, use one of your unixy virtual machines instead.
Also have a look at VMware Fusion. This is VMware for the Mac, and with its Unity mode you can access client menus direct from OS X, have client apps on the Dock etc. It's rather sexy.