The tagline at OldVersion.com reads as follows: Newer is not always better. That's true of Coke, Star Wars movies, and software. If you recently installed a new version of a beloved program, only to recoil in horror at its "improved" interface, "better" features, and "faster" operation, OldVersion.com is the site for you. It's home to a couple thousand versions of nearly 200 popular programs, from AOL Instant Messenger to ZoneAlarm.
Keep in mind that with some programs, older is not necessarily better, either. Even if you're not wild about the latest version of, say, ZoneAlarm, you don't want an outdated and/or unsupported firewall protecting your system — you want the latest and greatest.
That said, sometimes you just want to go back to the old, the familiar, the works-the-way-you-want-it-to. For example, I recently upgraded to version 3 of Copernic Desktop Search, only to discover it was missing a couple key features from version 2 (most notably dynamic searching in the taskbar). Alas, OldVersion doesn't stock Copernic, but there's a forum where you can post requests. Very often other users will come to the rescue with links to older versions.