NOKIA s60 Tricks

Silent Mode:
In some models it can be activated by pressing and holding the End/Red key. In others it's the # key.

Check running apps:
Press and hold the menu key to see applications that are running. Switch between applications and also stop applications by using the 'c' key.

Quick start applications:
When you open the menu or a folder in the menu you can use the keypad to quickly access an application The currently selected one is assigned to 5 key. The upper left corner is 1, etc. You can just hit any key and it will launch the application that's in the same location on the screen.

Customizing:
Besides the themes, you can also change the softkeys in the desktop and the quick launch application under Tools->Settings->Phone->Standby (the settings are somewhat different in different versions of S60)

Voice Commands:
Most Nseries recognize Voice Commands for dialing entries from the phonebook or starting applications. Just remember that you need to open the VOice COmmands application at least once for the recognition to be enabled. Press and hold the Right softkey until to hear a beep to start recognition.

Image (Picture viewer) application commands:
5 - Zoom in
0 - Zoom out
* - Full screen
1 - Rotate Left
3 - Rotate Right
2,8,4,6 - Move pictures (when Zoom level is high)
7 - (It does something but not sure what)

There's a ton more but I'll let other people share their own