I'll start by saying that if you don't have ATKF (Advanced Task Killer Free), you should get it. It's a godsend. This thread is nothing more than a compilation of my experiences with the program. I hope you can all find some use for the information herein.
I have noticed here (and all over the internet in general) the countless questions regarding ATKF, applications staying open after they have been closed, and an overall lack of memory. All of these issues, especially when they occur together, can drastically slow down the speed of your Droid making it difficult to do the simplest of tasks.
If you go to the top of the Droid screen and pull the tab down, you can switch to the ATKF program. From my experience, by clicking "KILL selected apps" the first time kills all of the current applications that you have opened recently and some that your Droid opens by default. Those programs that tend to open by default include Alarm Clock, Corporate Calendar, Messaging, and (sometimes) Settings. At this time none of these applications can be deleted or turned off.
*If you do kill any of these applications, the next time your phone locks or you pull up the applications tab from the bottom of your screen they will restart and you will have to kill them again. These applications tend to take up anywhere from 10-20M of memory.*
At this point, after you have killed those programs, take a second and kill ATKF too. If you restart the program you will almost always notice other programs that were open as well. I believe that there is either a limit to the amount of programs viewable by ATKF or some apps take precidence over others.
*In any case, a second run of ATKF will ensure that your phone is running at full capacity.*
All in all, after your Droid has been completely "killed" it should run at anywhere from 95-105M memory available. It is important to do this if you are about to play a memory intensive game (ie: Quake) or open multiple windows of any sort.
It took me a long time to figure this one out and it helped my Droid run a lot quicker. I plan to make a few more posts about some other topics. Once again, I hope this is of use