Most PCs can handle QuickTime videos, but playback for Windows Media (WMV) files on a Mac isn't always predictable. Flip4Mac WMV is a free collection of QuickTime components that lets you play WMV and Microsoft Silverlight files on your Mac, either through QuickTime Player or right in your browser. You can also pay to upgrade Flip4Mac for more advanced WMV features, such as importing, exporting, and creating encoding profiles, which integrate with other QuickTime applications like Final Cut Pro.
If you just want to play videos, you only need the free Flip4Mac WMV Player. Flip4Mac then installs plug-ins that let you play Windows Media streams and Silverlight media in Safari and other browsers. (You can also customize your installation if you don't want to install Silverlight.) You can change your settings in the Flip4Mac preference pane later, but for the most part this app's functionality is invisible once installed, with playback in your browser that's nearly indistinguishable from QuickTime.
Flip4Mac isn't perfect--for example, it can't play Windows Media Digital Rights Management, and it hasn't fully made the jump to handle Safari's default 64-bit mode in Snow Leopard. But if you ever visit the growing number of sites that use WMV and Silverlight (such as Netflix and its Watch Instantly titles), Flip4Mac gives you a free, easy way to watch videos on those sites.