Krebs on Security posted on several important software updates last week. (It’s a well-written blog, and very accessible to the average user. Highly recommended.)
Microsoft issued an Emergency fix for IE 6 & 7. Even if you don’t use IE, you still need this because some of the programs Windows uses (e.g. Windows Update) make use of IE.
Apple also published a pretty Huge Mac OSX update. Some of these security problems go back to 2003.
And if you use anything with Java (e.g. OpenOffice.org), there’s an update for that which covers 27 different issues.