It's a mobile world, but we have not fully abandoned the desktop PC. Real computing work (and a lot of the play) requires a full personal computing system. And to get the most out of it, you need software.

Software can be expensive, but free programs have been a mainstay of the desktop experience for decades, and today's offerings are powerful. You can find a great variety of free audio and video editors, office suites, file utilities, organizers, photo converters, and more. To keep things free, software developers can adopt an ad-based model, create donation-ware, or use a shareware/freemium model that charges for extra features.

To be included in this roundup, software must be available directly from the developer/creator/original publisher. It should (typically) have a Windows-based download—we're not including browser extensions, because we're not all using the same browser. But we've included web-based apps that are as good as—or even better than—most downloadable programs.

More requirements for inclusion here: If the software is on a tiered sales model, the free version cannot be trial-ware; it must offer at least a free-for-life option. Preferably, the program has had an update in the last year or two, and the program should have little or no advertising to support it. Finally, this list is about software for productivity; there are plenty of other places to find free PC games.

Always be on the lookout for crapware installers: To make ends meet, many creators of otherwise great free software, or the services that offer free programs for download, bundle in things you don't want—i.e., crapware. Worse, the installation steps are often obfuscated, so you might unintentionally provide the unwanted program with permission to be installed. For more about how to spot and avoid this problem, see How to Rid a New PC of Crapware.

A related pro tip: Download desktop software only from the maker of the software directly. It's not foolproof—after all, developers want to eat, too—but it helps.

For more free software, check out The 100 Best iPhone Apps and The 100 Best Android Apps.

Did we miss any free programs you can't live without? Let us know in the comments.