Only essential paid app for me is switchpro. It's a widget for home screen that adds toggle buttons for wifi/3g/flashlight etc..
Launcher Pro - replaces touchwhizz for the home screen (install paid version for some very good widgets)
UnBlockMe Free - game
Angry Birds Demo - game
Weather Forecast Widget - widget for home screen
Snowstorm - weather widget for home screen
Smooth calender - widget for home screen
CalWidget - calender widget for home screen
3g watchdog - monitors data useage
retro clock - nice data/time widget
es file explorer - browse network shares over wifi
dolphin browser hd - good web browser, but can be sluggish if not running a lagfix
barcode scanner - browse the appbrain or any other site for apps/widgets, then install them onto your phone by using the barcode reader to scan the QR bar code on your PC's monitor.
Also totally recommend apps organizer it's very very very good. Add a category to each app, then can add an icon for games that when you click it pops up a little window with all your games listed etc. You can also add a swipe action to the app draw icon that loads apps organiser and displays all the labels eg. games / tools / multimedia etc, to quickly get to any app on your phone.
My list here - http://www.appbrain.com/user/kiwijunglist
Also recommend lagfix by voodoo. If you install lagfix you must uninstall prior to any firmware upgrades etc. If you want to change the lock screen then widget locker ($1.99) is the best.