Sometimes new is not necessarily good. I distinctly remember a case where a customer, presented with a newer version of our software, demanded a downgrade - he liked the look & feel and stability of the older version.
This happens with downloaded software more and more often: add new features, take away the stability - that's the common trade off. But if you need an older version of a software, you may find that most download sites and the vendor's site only carry the latest version.
Enter VersionTracker, a site dedicated to preserving old software versions. You can download almost every available version of many shareware/freeware applications, some going all the way back to the year 2000.
The site covers Windows, Mac and Palm software. It also provides a service to get alerts when new versions of your favorite apps are available.
Update: my favorite download site FileHippo also provides up to 10 versions back on any app on the site. The selection, though, is quite limited, compared to VersionTracker's collection.