Here is a small tip to reduce memory usage of messengers whom used to work from system tray. Normally when you close a messenger window it will be minimized to system tray. Instead of closing it to system tray (clicking the x ), just minimize this to task bar. And if you want to use it from system tray then
1. Minimize the messenger to task bar
2. Right click and choose close.
If you want to test this here is the screen shots
1. Messenger maximized
2. Messenger closed(To Tray)
3. Messenger Minimized (To task Bar)
4. Messenger Closed( From Task bar [after minimizing] to System Tray)
This is applicable for MSN messenger, Gtalk ( I tested this only on these messengers, but I think this is applicable to all). If the application is having a facility to go to system tray when minimizing (like windows live mail) that is better than close to system tray.