There quite a few apps around that let you kill off selected apps at the touch of a button (Advanced Task Killer is the main one people use).
However there's quite a bit of debate as to whether killing apps off is a good thing.
As a developer myself, the fact the Andriod is using up RAM is a good thing. It allows fast access to apps. If RAM usage gets very low (I'm talking like <5mb at a guess), then it's probably worth killing off the apps. It's also worth killing off apps that use high CPU for no reason (most likely they are buggy and should be uninstalled anyway!).
Another reason to kill of the apps is to save battery power. You're only likely to save battery power if the apps in the background are actually doing anything (downloading/uploading data, or just CPU work in general). Most apps that support fetching data like this allow you to switch it off anyway.
There's also several apps around that show you how much CPU the apps are using (I use 'top'). Seems to me like very few (none on my phone) apps actually do anything when in the background (they have 0% cpu), so the battery usage won't really be saved.
Enjoy!