This thread will be updated as I add stuff, and feel free to suggest additions yourself!
ClockOnTop - top bar digital clock
Inbox Extender - add 'Delete All' and 'Mark All as Read' options to your inboxes.
MoDaCo DoNew - adds functionality to create new Word and Excel documents!
MoDaCo Google Search - changes Windows Live search in PIE and Windows Live to Google.
Palringo - Palringo, text, push to talk and image chat for your device. Install and join the 'MoDaCo' group!
PhoneSharpMT - Got a blog that supports an XML-RPC API (most do!)? Use your nifty keyboard to post to it with this application!
SKTools Lite - useful tools for cleaning registry, notification queue etc. Has additional options for freeing RAM quickly.
SKTSync - sync your device with a time server. Useful to install after you've just hard reset the device
Vito CopyPaste - adds cut and paste functionality. To install, copy EXE to device and run. To use, press and hold action key!