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New: PortExpert (Oct 12, 2021)
I think there should be an app called Steam Portable. That way people can play games on the go. IT'S REALLY USEFUL!
There was a version of it a while ago but the developer has stopped work on it, and for good reason.
There are far too many issues with it, especially the fact that a Steam Portable launcher will never be able to handle registry entries and filesystem changes for every game hosted on Steam, as there are far too many games already out there and more coming every week.
Also, you would bog your computer down for hours trying to run Steam Portable on a system with an existing Steam install depending on what games you had installed.
A program like Steam (indeed pretty much any storefront application) would be a mess to portablise. Not just due to possible conflicts between installed Steam and portable Steam, but due to the games themselves. As mentioned by Winterblood (a cool username I might add! XD) file system changes and registry edits would also be a pain to any portablisation attempt.
Portable Steam would have to have means of installing and running of games themselves, one would have to figure how to configure Steam (by default) to recognise the new PortableApps.com directory structure each time a game is installed or updated.
I agree that playing games on the go (from small indie games like Magicka, Bastion and Super Meat Boy to large commercial games like GTA IV, Half-Life 2 and Portal) is an awesome concept and I do use Steam frequently on multiple machines (despite the obstacle of Steam's login policy, 1 account online at 1 machine at any 1 time).
I am not necessarily saying it's impossible, just very unfeasible and complicated.
Most computers have a boot menu, if you want, use an Ubuntu Live CD/USB to install to an external hard drive/large and fast USB stick (WARNING: make sure you set the boot-loader to install on that drive as well as the OS or you'll screw up the computer you're using)
once installed, go to school/work and plug in your hard drive and access the boot menu (Dell: F12; HP: Automatic if plugged in and reboot, wait a while as HPs take a long time to show bootloader after boot, but it does eventually) and start Ubuntu. You can then install the Steam for Linux beta or use Wine/Crossover to get you full library of games (Wine/Crossover are buggier but more games are accessible).
Disclaimer: I'm not responsible if you screw stuff up. Also, Steam for Linux is in beta so there are a few glitches (console shows up and you have to accept installing stuff from there on first use) and there is only a small library of games for it (TF2 works, but FAST hard drive highly recommended). They came out with Linux Steam because Valve has plans to release their own Steambox, but they don't want it running Windows due to licensing, and the CEO hates Windows 8.
If there are not enough games on the official Linux Steam, use Wine or Crossover to run the Windows version on Linux.
I'd be more interested in getting some Steam games to run on a portable hard drive. Don't care about portable 'Steam' per se' and don't want the whole thing installed portable.
Something to dupe the game's exe into thinking steam is installed is all that's needed.
Some Steam games will work fine without Steam itself running, but I would presume that where a game does require Steam to be running it would be against their terms of service to bypass that.
Valve doesn't want there to be any different versions of Steam. A while back, a person created a *working* version of Steam Portable, but Valve had it removed.
[Link to illegally repackaged software removed by mod Ken Herbert]