Steam game files portability

Option one - Use backup/restore option in steam itself to create installers that you can then put onto an external drive and restore on your desktop.

Option two - Copy the specific game(s) folder(s) from your 'steam/steamapps/common' folder, and put them in the same folder in the steam installation on your desktop machine. Then select to install those games in steam itself, and it should find the files you copied to the machine and install from there rather than downloading from the internet.

For one or two games the backup/restore method works fine, but I've had problems with it when copying a lot of games at once, plus it can be slow to compress the files into a backup. I have terrible internet at home so I tend to download most of my steam games on my laptop at a friend's house then use the methods above to get them onto my desktop machine, so it should definitely work for your setup.

I also recommend these methods.
Can't you just install steam and steam games on you external drive and play games from there? :D
We have 3 mbps at home at it's a common download speed around here. 20 mbps is not slow at all. You can download 9 Gb in an hour. You can use the steam settings to allow download during game and play some single player while you are waiting or just go and take a shower. These processes also take time. Get to your friend, backup, restore and stuff.

It's okay! You can bring your files with me! I'm used to Poké Balls so external hard drives should be fine.

Steam files are nicely portable for similar operating systems. The directory structure:
Steam/SteamApps/:
----appmanifest[underscore][numbers].acf
----downloading/:
--------[games you are currently downloading, avoid messing with it when it's not empty]
----common/:
--------[game name]/
------------[game files]
------------steam_appid.txt

To copy a game to another computer you need its appmanifest[underscore][numbers].acf file and the common/[game name] file. If you know the game name, you can look in its steam[underscore]appid.txt file to find the right value for [numbers].
(I seem to be having trouble putting underscores in some places. Replace [underscore] with an explicit underscore)

It is possible. Copy the whole game folder from the directory '...steam/steamapps/common/GAMENAME' and paste it on your PC in the same directory. Start steam and click on download. Steam will verify the files already there and download any missing files. It should be done in a minute or two, and you are good to go. Also, I think steam says downloading, when it is in fact checking the files.

If you use Windows, then just here's what I suggest... if you have a network cable or a wireless router, bring the laptop to your house where the desktop is and use a FTP server on the desktop, then use a FTP client on the laptop to download the files directly, fast and easy.

About the files... just grab the SteamApps and UserData folders... or you could just grab the entire Steam folder for easyness' sake.

I personally use FileZilla as FTP server between my laptop and my desktop (the portable alternatives because I like portable programs).

I will try the 2nd method r00d has suggested here before moving at the beginning of the next month and see how it works. I do not have a laptop, so will have to just try doing it at my friend's place and bring it in external hard drive. I am not going to install the game on the external drive as someone has suggested. I know how bad can that be. Will update the post about the results to let you guys know how it went with me.

Thanks a lot for all the input. This is why I love this community. Given that there are some people who are kind of assholes, overall this is a very good community.
:) :)

I download all my Steam games, archive them, then uninstall the games that I won't be playing soon.

When I want to reinstall, I just extract the game to its original location, then tell Steam to install the game. Steam says 'discovering preexisting files' before downloading anything, and does not redownload anything that is already perfect.

I save a lot of download time this way, and keep my internal hard drives lightweight.

Well I just use the built-in Backup and restore system that Steam has. It is a little buggy though, most of the time when you try and select multiple games to backup it'll screw up, so do one at a time, 98% of the time this works flawlessly. I have tried copying the files from the Steamapps folder and Steam ended up downloading the games again for reasons unknown, so I'd go with the Backup/restore system.

Steam does support running from an external disk as of the beta, if I remember correctly - I saw some option that let you manage install directories or something. However, this is guaranteed to lead to really, really long loading scenes and impossibly slow elevators.

As for just copypasting the game files over, I'd suppose this would work, just make sure to validate the files afterwards and hope you don't have any missing textures.