Memtool is a new task manager for your iPhone/ ipod touch available in Cydia via the iSpazio repository from developer NetDragon. When you fire up MemTool the default page you see shows you a graph with the total memory available (128mb) and the memory in use. By pressing the Clean button the application will attempt to free as much ram as possible.

The second tab on the application gives you the option to kill processes individualy by memory usage of by name.
See screenshots: