With its multi-tasking, it's easy to launch lots of applications on Android. I understand that background applications use fairly minimal resources, but they often activate features (GPS WiFi etc.) that drain the battery.
How can you find out what applications are running and what resources they're using? and how can you stop them safely?
All Android applications have to be granted 'Permissions'. These 'Permissions' include internet access, GPS, etc.
You can see these 'Permissions' by going to the application in the market, hitting 'Menu' and selecting 'Security' (I think).
As for 'activating features', applications cannot activate GPS or WiFi if you have them turned off. Also, GPS, even when activated, is only 'awake' when an application is polling for location, then immediately goes back to sleep.
It's always good to remember to turn off WiFi and other features you don't need when you aren't using them, although Android is very good about only using these features when necessary.
If you go to
Start screen / Menu key / Settings / About phone / Battery / Battery use you can see exactly how much different applications drained your battery since your last recharge.
To stop application you can use the file manager ASTRO or a Task killer like Advanced Task Killer.