Open up your Macintosh HD and at the bottom of that window it will tell you how much free disk space is available. Otherwise, if you highlight the Mac HD, and go to the File menu, Get Info, it will tell you everything.
Leopard itself can take up 10-30gbs or so, depending on if it's the stock install (which has extra language packages), or if you installed the extra loops for GarageBand, so on and so forth. So it can add up quickly.
If you use bittorrent, it can add up even quicker. If you have iTunes saving and using its own library (default setting) then you could have duplicates laying around from downloads.
You could have one of those run-away log files that isn't properly deleting itself in which it keeps bloating up and eating space. Run an app like Cocktail or Onyx to delete all log files.
Even if you have "enough" space available, you typically want like 10-20% free for system/application use (if not, then a couple gigs free is always nice to have) on your boot drive.