Killing apps/services in Android is generally a really bad idea. Whilst it is possible to write "task killer" apps, it shouldn't be encouraged for anything outside of development/debugging purposes.
Task management is the responsibility of the Android O/S, the tasks you can see are not processes (in the sense of the processes you see in the Windows task manager for example), in fact, they only have a process when Android tells them they can have one.
Apps are regularly broken by these task management tools, as they often fail to recover from the forced termination, particularly if they were busy writing to a file or using another resource when they were killed. It also puts the handset users into a false expectation that the apps listed are actually RUNNING on their phone, which they are often not. This is explained in the ActivityManager docs:
Information you can retrieve about a
particular task that is currently
"running" in the system. Note that a
running task does not mean the given
task actual has a process it is
actively running in; it simply means
that the user has gone to it and never
closed it, but currently the system
may have killed its process and is
only holding on to its last state in
order to restart it when the user
returns.
When you see the list of "running" apps in apps like TaskKiller or Quick System Info, many of them are not actually running, they are just in a suspended state. These apps are not consuming system resources because Android has decided to stop them until they are needed again. However, when you kill them, you don't give them time to shut down cleanly, and when you try to launch them next time you can be presented with an unfriendly force close dialog. I have seen apps break completely, with even a re-install being ineffective, because they are trying to read a corrupted file on the SD card, or they use unofficial API calls.
In short, friends don't let friends use task killers in Android.
Anyway, to answer your question, the ActivityManager is what most of these apps use to list activities that are in running/suspended state.
This is an example of one of these "task managers" in use:
http://code.google.com/p/freetaskmanager/source/browse/