How do I view open applications?

Hi, sometimes when I have an application open and I press the wrong button I might accidentally close it (or at least the application looses focus). But on many instances it seems that the application is still open - so that when I either try to reopen it, or open another application, I will get a message saying 'out of memory'. So my question is, how can I view which applications are open when I do this, either to bring them back into focus, or to close them?