K9 is a better email client than the stock one
Plume is a great Twitter client
LauncherPro replaces the Home screen very nicely
Touch Calendar is better than the stock calendar
Auto Rotate Status Bar Switch is very handy for reading on the phone while lying down, as it stops the phone changing from landscape to portrait (or vice versa)
Upvise is a good shopping list manager (Julie enters things from the web, it syncs to my phone, I buy them on the way home)
MoveToSD is a quick way to move apps to the SD card
Swype is a better keyboard than you'd imagined could exist on a touch screen (not available through the market, you have to sign up for the beta on their web site).
Handcent SMS is superior to the built-in SMS client.
Anything I'm missing that I should have?
Original post on Dreamwidth - there are comments there.