Minecraft is the hit indie game by Notch that is still in alpha but is already making waves, thanks to a free-to-play older version that runs in your browser. It's a sandbox game that puts you in a world of oversized voxels and challenges you to mine for resources, craft those resources together into tools and supplies, build structures above and underground, and make Minecraft's world yours. Oh, and watch out* for creepers.
* - Mildly NSFW, language at the end
Download Minecraft Portable 1.1.2_01 Development Test 2 [0.4MB download / 4.5MB installed]
184.108.40.206 (2010-10-14): Fixed a bug where it wouldn't work at all. HURR.
220.127.116.11 (2010-10-10): Initial release
This release includes launchers for both client and server. The server can be run by anyone, but the client requires purchase of Minecraft from minecraft.net for 9.95 EUR (~US$13).
The installer itself will not install Minecraft or the server; the launcher will download whichever one you're trying to run from minecraft.net the first time you run it and log in, so be sure to run both launchers (if you want to use both; otherwise just whichever one you want) first on a computer with an internet connection. This is done in order to attempt to comply with the Minecraft EULA as well as give Notch his requested control over content distribution.
Install size increases with extra resources; painterly pack, saved worlds, etc. Worlds are usually not too big but be careful since they have tons of small files; the size reported by Minecraft will not be accurate due to the way filesystems work. You can use the "Size on disk" field in the Properties dialog of a folder to see the real space consumed, but be aware this will be different for different disks (and may be a lot higher for FAT32 thumb drives than fixed NTFS drives depending on what we nerds call "cluster size").
Client Minecraft engine and resources, and saved worlds for clients and servers are stored in Data. The client launcher, and the server engine and resources are in App (this is due to differences in how Notch packages the client and server; client engine and resources have to go together with the saved worlds).
If you receive a launcher out of date message from Minecraft when trying to launch it, remove App\Minecraft\Minecraft.jar to force Minecraft Portable to download the most recent version the next time you run it (or just reinstall Minecraft Portable which will delete it for you). I don't believe the server will report it is out of date but the same applies there too with MinecraftServer.jar.
The codebase for this is based off of a stripped Google Chrome Portable launcher, plus the core of the Java Launcher. Really, the Java Launcher in itself may be enough to run Minecraft portably, but I think my solution is a bit nicer.
I'm sure others have probably already posted Minecraft launchers here, this is just my take.
Oooh I just had a great idea, a DefaultData with a PORTABLEAPPS.COM logo made in Minecraft! BRB...