Where can I find my Android applications?
The /data/app-private directory is another location of 3rd-party protected apps.  System apps that come pre-installed with the ROM are stored in a read-only /system/app directory.

/data/dalvik-cache contains optimized bytecode for each installed app, which allows the system to launch it faster.  These ...@classes.dex files get generated at installation/update time, or during the first boot-up for pre-installed apps.

Also, with Android 2.2 Google introduced ability to move apps to the SD Card.  When you do this, a new virtual mount point is created under /dev/block/dm-X (where "X" is a sequential number starting with 1) which points to /mnt/asec/xx.yy-1.

/data/data/ folder contains settings, preferences and cache for all apps, each within its own folder named after the package name.  E.g. /data/data/xx.yyy/.