Ever had a client who requested that their site be compatible with an older browser, but you can’t test it because you don’t have that particular version of the browser in question? Here are a couple of sites that archive old browser downloads, and I can tell you that they have already saved me loads of time. It’s hard to know how the browser wars will play out, and it seems inevitable that support for legacy browsers will not continue indefinitely, but for now we still have some help:
Evolt is the king of browser archives. Their list is very complete and with hundreds of mirror sites – you are sure to get what you need.
Netscape also maintains an archive of its older versions.