WMV for Quicktime
If you need to see WMV files on Mac OS X, until recently you had to use the Windows Media for Mac.
If you knew that and used it, you know that it wasn't that good.
Recently, there where some mentions about Flip4Mac on Mac OS X Hints, and this week, Microsoft said that it would stop developing the Media Player, and made and agreement with Flip4Mac to distribute their WMV Quicktime plugin and standalone player software for free.
I've downloaded a version last week, and installed. In the next few days, I experience crashes in both Mail.app and Safari, something that I hadn't before. I decided to reinstall and yesterday I downloaded again. I don't remember which version I downloaded the first time, but yesterday I picked up 2.0.1 and since then the crashes have stopped.
I don't know if they released a minor upgrade due to some problems, but if you are having random crashes since installing Flip4mac, try uninstalling (there is an uninstaller in the /Applications/Flip4Mac folder), download the 2.0.1 release and install again.