If you're using a portable Mac, consider building separate libraries on both your internal and external drive. This gives you access to your entire library while your machine is plugged into the external drive at home or work, and you can also have a small subset of music or other media -- such as movies to watch on a plane -- with you at all times. Tools like Syncopation (US$25; free trial) and iPhoto Library Manager (free; advanced version US$20) can help you manage this dual-library existence.
Another option that has both organizational and disk-saving options for iTunes and iPhoto is to search for duplicates in your library. With thousands of songs and photos, having duplicate tracks or photos is a very real possibility.
Both iTunes and iPhoto provide basic duplicate detection features, but those features may not always turn up all your duplicates. iTunify (US$15; free trial) and iSweep (US$15; free trial) provide advanced duplicate detection for iTunes, and Duplicate Annihilator (US$8; free trial) provides in-depth detection for iPhoto.
4. Clean out logs
I mentioned this one in my recent list of tips for keeping Leopard purring, but it's worth mentioning again. Log files are generated by a number of Leopard's processes as well as by applications, which may maintain their own logs or record items to Leopard's system.log file.
Leopard's maintenance scripts automatically archive and compress log files on a regular basis. Even so, the number of archived log files can grow rather large. If you do not have a need to keep archived logs dating back weeks, months or even years, then you can remove some of these older log files to recover some disk space.
Systemwide logs (those that record events from system components and applications that impact all users of a computer) are typically stored in the /Library/Logs folder at the root level of your start-up drive, and user-specific logs are stored in the /Library/Logs folder inside each user's home folder.
5. Delete unused applications and tools
This one may seem like a no-brainer, but it's worth mentioning because it's easily overlooked but can really affect performance. Deleting unused applications (including those that come with Mac OS X, those that come with other commercial suites or shareware apps you've downloaded but never really used) alone can save a lot of disk space.