Adobe has released a security update for its Flash player, which you almost certainly have if you've ever watched any video or animated image online. The update patches nine potential security holes.
To check which version of Flash you currently have installed, visit this page:
http://www.adobe.com/products/flash/about/
Assuming you're not up to date, you can download the latest version by clicking on the "Player Download Center" link, right on the same page.
I just did it. The download is virtually instantaneous (over broadband), and the installation takes about ten seconds: close your browser and double-click the file that you just downloaded. You won't even have to reboot. So go do it, right now. You can then revisit Adobe's site to confirm the success of the installation.
Brian Krebs has more details, if you want them.
P.S. In the same post, Krebs also notes that Opera has released a security patch for its browser, as well. I don't use Opera, so I don't know anything more than that. I'd be interested to know if Opera has an automatic update/notification feature like Firefox, so Opera users, please drop a note in the comments. Thanks.
P.P.S. In an earlier post, Krebs reports that Apple has released a bunch of patches for Tiger (v 10.4, not Leopard - v10.5). I don't have my Mac hooked up right now, so I can't say anything about this one, but I imagine the usual Software Update procedure should handle things. NB: This one looks like it might take some time, especially if your connection isn't speedy -- one of the patches is an 80 MB fix for security holes in Java. But don't put this update off for too long. According to Krebs, there are 58 security vulnerabilities being addressed, and apparently, Software Update fails to impress upon you the seriousness of some of the Java security holes.
Happy surfing!