As far as killing tasks to save battery I use Advanced Task Killer. I just have the widget sitting on my main page that I can hit everytime I unlock the phone. I think its automatically set to kill background tasks anytime the screen goes off and the phone locks.
I havent played around with browers much, I always just use the preloaded one and havent had issues.
For a launcher (like the Go Launcher mentioned above) I use Zeam Launcher. Its basic, simple, and uncluttered, and does not take up a lot of memory. Its just customizable enough for my liking. Its laid out similar to ADW Launcher that I was previously using on my last phone. I havent tried Go Launcher yet.
For an overall utility to explore the contents on your phone, I have ES File Explorer. Think of it like windows explorer. I use it to access files that I have loaded on the phone with my USB cable, say, a PDF file or something.
If you play Forza Motorsport, theres an app called Forza Droid. Input a little data about a car you are building, and it spits out a tuning setup for your car. I havent used it with Forza 4, but I used it with Forza 3 a lot (it was really made for Forza 3). If anything it gives you a decent starting point for getting your car set up.
Air Control (you can get a free version) is a super addicting, time killing game. Its been out for years but still probably my favorite.