While there are many choices available in the Android Market, there is one that in my opinion outshines all the rest. Super Task Killer FREE by NQ Mobile (Netquin Mobile Inc.) seems to be the most efficient out of all which I have tested. It includes a simple 1x1 widget which is easy to fit on you home screen, and kills apps with a simple tap, and will flash a notification screen giving you a report on the amount of memory available.
There are times that you may have to press the widget more than once to kill everything, but typically this is not nessacary.
As with all task killers, you can expect certain applications to restart automatically, as they are required for your android device to function normally. This is best used after running several third party apps, such as StumbleUpon or a Craigslist browser since they will likely stay running in the background.