TaskSpy has been an excellent freeware task manager for Symbian S60 and UIQ based mobile devices. It allows you to get information and manage running tasks on your device.

Featuers include:

-quickly switch tasks
-kill processes and threads
-free unused memory
-display detailed information about system resources
-track cpu load and free memory

Unfortunately, TaskSpy is available as an unsigned application for developers only who can then sign it with a DevCert. Else it will not work as the software has to access the system which is restricted on S60 3rd Edition platform.