After 18 months or so with OS X, I've gotten to where I have a pretty good feel for what I like and what's missing – at least in my view. To bend OSX to my style, here's a list of my favorite OS X add-ons:
Letterbox for Mail.app – an outstanding widescreen, three-panel interface mod. This may be the best usability add-on out there.
MenuCalendarClock – iCal integrated menubar calendar. Why isn't this a default app/option? Jeez.
Growl – Sometimes I forget this is an add-on, because it is so tightly integrated with so many of my favorite applications. Growl gives notifications from applications & system events without changing the focus.
MailPod – Mail.app add-on which allows you to send messages to a folder on your iPod. The more you work with filtering/smart folders, the more you'll enjoy this one.
MailTags – I was late to the "tagging" game, but tagging emails makes them *very* easy to find with search or Spotlight – think receipts, reservations, etc. You can also tag items for follow up in iCal – very cool.
GoogleMaps for Address Book – One click access to Google Maps & directions for Address Book entries.
Todos – Every app at your fingertips. Todos is basically a visual program launcher, instantly available via hotkeys.
RCDefaultApp – Adds a preference pane to the System Tools that allows you to specify your default applications (browsers, mail programs, etc) and actions (launch "x" when camera attached, etc).
Witch – I think I got this one from Josh, but it is one of my favorite add-ons. Witch is a hot-key driven window manager that allows you to switch from window-to-window for all open apps, instead of just app-to-app a la Expose.
Meteorologist – I'm a sucker for weather apps, and Meteorologist is a great menu-bar tool for multi-location weather (ie my zip & the beach house!). It interfaces with multiple weather services, gives current weather, forecasts, alerts, and more.
SlimBatteryMonitor – A great menu-bar tool for laptop battery status (or UPS battery status, if you're so inclined). I prefer it over the Apple tool because it is much, much smaller & customizable.