Stacker allows you to compress your applications when you aren't using them. Now available for the P800, this winner on the S60 and 9210 is available for all Symbian OS phones.

From the author:

Stacker is the best, fully automatized and reliable compression system for your device. Stacker software doubles the size of your entire drives and works invisible behind the scene. When you start an application it becomes decompressed in background automatically and is compressed again when you close it.

Stacker is the best way to increase disks space on your device, it guarantees to give you as much disk space as possible without even thinking about it, moreover Stacker includes fully featured file manager with compressing/decompressing files or folders functions.

You can download the trial here.