Although Tim is correct, I believe, about closing truly "open" (i.e. running) apps, you can free memory with an app called "Xsysinfo." I just ran the app and my "free" memory went from 8mb to 300mb + on my iPad 2. And I suspect that is what you're trying to do.
A couple of points, though.
First, you really should not have to "free" memory devoted to background apps manually. iOS is designed to do so on its own. (This same debate, by the way, goes on and on in the Android world, as well.) But having said that, I've run into several cases since upgrading to iOS 5 where clearing memory has resolved a problem I encountered. For example, I could not access app descriptions in the App Store yesterday. Cleared memory manually and the issue disappeared. I've had the same result with several other app crashes and cases where an app behaved in an unexpected manner. These problems have occurred on my iPad 2 only since upgrading to iOS 5; I think they result from some unresolved bugs in the new OS.
Second, Xsysinfo was apparently much more useful among owners of the original iPad (where memory was only 256K); it has been less useful for those with the iPad 2. At least until the upgrade to iOS 5 as noted above.
Third, Xsysinfo was modified a couple of months ago at Apple's request to disable the "free memory" option. That more or less crippled the app and made it pretty worthless. However, a subsequent update restored the functionality apparently in accord with Apple's requirements. The "free memory" option, however, is no longer as obvious as it used to be. Now, rather than pushing a button, the user drags the "usage" panel down to activate the "free memory" option. I think many of the app's former users are not aware that it has had its functionality restored.
Bottom line. Xsysinfo is available in the app store for $.99. And though the description no longer mentions that it will free system memory, it does. And there may be other products that accomplish the same thing. I just don't know about them.