Windows 2000 mainstream support is due to end today and a couple of days back, as expected, Microsoft released update rollup 1 for Windows 2000 service pack 4. Full details of this update (including why it is not called Windows 2000 service pack 5) are included in Microsoft knowledge base article 891861 and the problems which it resolves are listed in Microsoft knowledge base article 900345.
I've been working with Windows 2000 since the late 90s, when it was NT 5.0 beta 2, and I guess I'll still be using it for a while yet (as will many of my clients) but for a view on why 48% of corporates are still using Windows 2000, see my decision time for Windows 2000 users post from a few days back.