If you're installing apps directly on your iOS devices and if you're jailbroken you can check:
This directory contains copies of downloaded apps. They should be deleted once you synced your iOS device using iTunes, but if for some reason a sync would fail, they might end up staying there until the next restore.
I had about 3 Gigs there before I deleted them...
[EDIT] Make sure you sync all purchases from your iOS device to iTunes BEFORE you delete those zip files!
Another reason for the OTHER space are jailbroken apps and their data, e.g. downloaded movies, comic books, etc.
And a third reason might be lost music/videos. To get rid of those you have to remove all your music/videos in iTunes and then check the directory /var/mobile/Media/iTunes Control/Music
delete every thing that's still in there and then others should be tiny again.
[EDIT] You then have to sync your music and videos back again.
Of course you can always restore and restore from backup.