Written by Kevin Purdy

Whether your important data lives in the cloud, on your laptop, or on a different operating system, you shouldn’t have to use sub-par tools to get at it. These downloads work with every major operating system, along with some not-so-major (mobile) ones.

Photo by Mykl Roventine.

All of these applications run on all three major operating systems—Windows, Mac OS X, and Linux—and most can be loaded onto a thumb drive and run as a portable app on any Windows system. Some can also be accessed from the web, and a few have dedicated mobile apps for most phone platforms. We’ve distinguished which apps work where at the front of each item. If we’ve missed any platforms, please tell us so (politely!) in the comments.

10. Buddi

Computers: Buddi is a financial management application developed with financial non-experts in mind. Sure, it can import your CSV file from a bank or financial firm, and it does all the standard financial calculations and projections. But the way it switches between money figures, and walks you through the importing and setting up of your accounts, makes it a real open-source find, and you can easily swap profiles between your laptop and desktop systems, if needed. Looking for something with a bit more mathematical oomph? Money management alternative GnuCash has you covered.

9. KeePass

Computers, portable, cellphones: You use a multitude of applications and web sites that require passwords, license keys, and administrator codes. On one computer alone, that makes it worth having a central vault for all that stuff. If you use more than one computer, having a consistent KeePass database is really, really helpful. Encrypt your master password database with a file only you have access to, and/or a truly secure single password, and you can take that list just about anywhere—on Windows, Mac, Linux, iPhones, Android, BlackBerry, Palms, on a USB drive, or pretty much anywhere. Open-source coders love to write KeePass apps, so there’s a very good chance you’ll always have this clever password management system at your side. For help getting started with KeePass, check out Gina’s guide to securely tracking passwords. (It also works great in conjunction with Dropbox.

8. TrueCrypt

Computers: TrueCrypt is a multi-platform security tool for encrypting and protecting files, folders, or entire drives. The software behind it is open source, and so likely to be supported and developed beyond its current version and platforms. It’s only on Windows, Mac, and Linux at the moment (though that’s no small feat), but it can be made to run as a portable app, and its encryption standards—AES, Serpent, and Twofish—are supported by many other encryption apps that can work with it. In other words, TrueCrypt makes you feel better about taking all the revealing information about yourself or your work on the road. Check our guide to encrypting your data for more.

7. Thunderbird

Computers, portable: Mozilla’s desktop email client is an excellent tool for reading, sending, and archiving email, even if it doesn’t get a ton of love these days—seeing as how seemingly everyone’s doing their email thing on the web. But even if you don’t use it as your main email client, Thunderbird remains the most reliable way to back up your email from any service and, in most cases, still access it when the web interface goes down. With the imminent release of Thunderbird 3, and the portable version to follow right after, Thunderbird might just turn a few more folks back to the idea of desktop email.

6. Pidgin and Adium

Computers, portable: They’re not the same program, but they come from the same open-source roots. These instant messaging clients do the yeoman’s work of connecting to all the major chat protocols and helping you maintain a universal buddy list. Pidgin does the job adequately, if without a ton of pizazz, on Windows and Linux clients (you can spice it up a bit with these snazzy plug-ins), while Adium, compiled from the same libpurple code library, is written with OS X’s glassy looks in mind. Both are crucial if you don’t want to run multiple memory-sucking IM clients on all your machines.

5. Miro

Computers, portable: Miro doesn’t get enough love (here or elsewhere) for being a pretty great all-in-one aggregator for all the video on the web. The open-source video player handles video podcast feeds, Hulu streams (which you can subscribe to, show-by-show, TiVo-style), live streams, local files, and anything else with moving pictures with ease and grace, and you can take it wherever you go to ensure you can watch your favorite web-accessible or desktop videos.

4. 7-Zip

Computers, portable: 7-Zip doesn’t have the sexiest job on a computer, but since no two operating systems accept all the same compressed file formats, it’s an essential download. It tackles the RAR files that file sharers are so fond of, makes sense of .tar and .gz files on Windows systems, and has its own compression format (.7z) that’s space-saving and quick.

3. Firefox

Computer, portable, and (coming soon on non-Maemo devices) mobile: Even if you don’t think it’s the absolute fastest or most cutting-edge browser, Firefox is safer than the well-known standard on most Windows systems, and it’s customizable in every last detail. That makes it worth keeping on your USB drive as a go-to option for browsing at the in-laws or at home. With add-ons like Xmarks or Weave, it’s also easy to keep your bookmarks—and keyword bookmark searches—within reach on any system. And when Firefox Mobile, a.k.a. Fennec, makes its debut on mobile phones, we might see some rather awesome synchronization of everything, right down to the last tab you had open at home.

2. Dropbox

Computers, web, mobile: Dropbox creates a single folder that you’ll always be able to access, no matter where you are. That folder can actually sync files and folders from anywhere on your system, but the concept remains the same—instant backup for anything you drop in one location, across multiple computers, through Dropbox’s web site, on the iPhone, and on mobile browsers. That makes it perfect for music you love to listen to, documents you need to work on, and photos you pick up at a relative’s house. In other words, feel free to stop emailing yourself.

1. VLC Media Player

Computer, portable: Managing the multitude of codecs, formats, and restrictions on media files, from one system to another, is a pain you don’t need. VLC Media player, installed on any system, just works. It’s built with the goods to process, convert, resize, and stream just about any file you can find with audio or video, and its presence on a USB drive ensures nobody ever comes up embarrassed when their nephew’s soccer video just won’t play, even though, they swear, it worked just yesterday. For a guide on making the most of VLC’s cross-codec powers, read Adam’s tips on mastering your digital media with VLC.

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What apps are always with you, or always downloaded, when you’re switching between systems, traveling, or otherwise away from your preferred setup? Do tell us in the comments.