Here's my 5 (+1 for good measure) in no particular order.
Switftkey X: Swype started annoying me a couple months in. Switched to SwittKey; SwiftKey X is just better.
TuneIn Radio: I don't have a car, so it's nice to still listen to the radio. And it saves me from having to buy ESPN Radio app.
Springpad: I use it for everything. Keeping track of movies I wanna see, blu-rays I wanna buy, video games I wanna get. You can add tasks, notes, recipes, scan products. It's a chaotically organized system of stickies. And also a decent companion to the desktop version. I think the app plays off well with the way I remember things.
Dolphin Browser HD: So good.
Fancy Widget Pro: I like the way it looks. Has customizable backgrounds and weather skins. Can link to 3 other apps by just touching different areas of the widget.
GO Launcher EX: I didn't like TouchWiz, and Launcher Pro was too plain to me. Go Launcher is constantly updated, includes a lot of cool options, allows you to customize app drawer, and has a lot of free themes. They've gotten better about speed and memory issues, and they are constantly adding improvements.