task panel, bar none, best app. It's totally free.
Here's why I love it.
Task Switching
When you start it, it shows what's running, tapping an app takes you to that app.
Task Killing
When start it, it shows what's running, tapping and holding brings up choices like killing the app, adding it to the Ignore List, and adding it to the Wanted List.
Additionally when it starts it always has two standard options with the list of running apps, they are KILL ALL and BACK TO HOME SCREEN. Very convenient.
Ignore list
You can add things like the alarm, the keyboard, the dialer, and any other apps you want to NOT kill if you use the KILL ALL option.
Wanted list
This is my favorite feature, one that is missing from all the other task killer apps I've seen so far. You can have Task Panel check available memory every so often. If memory is below a threshold that you pre-determine, it will start killing apps listed in the Wanted List. For example, I never use Voice Dialer or Voice Search --- but for some reason they pop up on their own in the list of running tasks. I added them both to the Wanted List so that they don't lag my memory all the time.
This isn't a foolproof feature, you have to be smart about what you add to the Wanted List. For example, I had 3rd party email app I use for work on the wanted list. The problem was that a couple times when I was trying to use that app, the memory theshold dipped and it was auto-killed. Obviously some apps shouldn't be on the wanted list.
In the past I used task killer and task switcher, and both had things that made them less than perfect.
I used DG Quick cut to put Task Panel in the Notification shade, so that I have easy access to it along with toggle settings and memory for lemmings.