Have a look at how space is being used:
OmniDiskSweeper (free) - http://www.omnigroup.com/applications/omnidisksweeper/download/
OS X Lion: About Time Machine's "local snapshots" on portable Macs - http://support.apple.com/kb/HT4878 - “You may notice a difference in available space statistics between Disk Utility, Finder, and Get Info inspectors. This is expected and can be safely ignored. The Finder displays the available space on the disk without accounting for the local snapshots, because local snapshots will surrender their disk space if needed."
Hidden local backups with Mac OS X Lion filling up your MacBook hard disk! - http://blog.valerio.de/post/19238158977/hidden-local-backups-with-mac-os-x-lion- filling-up-your - Time Machine using local drive to store backups when external drive is not available.
Of course it may be that your hard disk really is full of movies and photos and music and you need to clear some off as soon as possible or risk a crash.