Sooner or later it happens to every Mac user ... you encounter a file of some sort that requires a piece of Windows software to open. Thankfully, this happens much less than it used to. Most of the files people exchange these days cross platforms easily: photos in the JPEG format, songs in MP3 format and Microsoft Word, Excel and PowerPoint files for which Mac users have several options.

But video files can be a bit more challenging, as there are a multitude of formats. Fortunately, Mac users can play almost all of them by adding a few free components to their system.

Of course, the primary means of playing video on a Mac is Apple’s own QuickTime Player, included on every Mac. The QuickTime format comprises a relatively small portion of the video you’ll encounter on the Web, but the player handles several other formats, too. However, with a free utility called Flip4Mac, the QuickTime software can play any Windows Media 9 Player file unless it’s encrypted with digital rights management (DRM). Nevertheless, Flip4Mac works seamlessly in most cases. Microsoft actually endorses it on its Web site, no doubt delighted at the excuse to drop development of Windows Media Player for Mac OS X.

One other format you’ll encounter from time to time is RealPlayer, which last week released version 11 of its software, so far for Windows only. The Mac OS X player (another free download) is still on version 10; here’s hoping Real upgrades the Mac version soon. RealPlayer has lost popularity to Windows Media Player, but it’s still commonly used by many Web sites for streaming video.

Adobe’s Flash Player, once confined to displaying animations, has also become quite a force in Web video, chiefly because You Tube uses it. It’s probably already loaded on your Mac; if not, it’s a free download from Adobe, available in both PowerPC and Intel flavors. You probably should download it in any event just to ensure you have the latest version. The software works within your browser, as QuickTime does, so when it’s working properly you’ll just see the video appear on the Web page in its designated window.

But even with all that video software loaded on your Mac, you could run across a video file that just won’t play. For that you’ll need the VLC Media player, a sort of Swiss Army Knife of media players. It resulted from an open source software project called VideoLAN, and thus is full-featured and free to all. It supports newer formats, popular existing ones (like WMA and QuickTime), but most importantly it supports less common or older formats such as DivX and Cinepak. Generally speaking, the VLC Media Player can open any video file not crippled by DRM.

Just for good measure, the VLC Media Player can also open most audio formats. As a utility of last resort, it can’t be beat. It’s a good idea, however, to check frequently for updates as the folks who work on it often fix bugs and upgrade the code.