Download and Install Dropbox
Here is a quick tutorial on how to migrate your saved games to the cloud so you can play your levels wherever you may be.
The first step is to download Dropbox. Dropbox is a nifty little tool that automatically syncs content to their servers, and allows you to access it both online and through a client on your PC. The good thing about Dropbox is that it is cross platform, plus available on the web and web-enabled smartphones. Go to http://www.dropbox.com. You get 2GB for free automatically.
Once you have installed Dropbox, you'll need to know where the folder is that it will sync from. On most Windows machines, it is located inside the "My Documents" Folder.
On Linux, the default location is in the user's home folder. For example in Ubuntu, that folder is located at ~/Dropbox
Whichever operating system you are using, Dropbox has very good documentation and can assist you in finding help: http://www.dropbox.com/help
Copy your Minecraft saved data to your Dropbox folder
Next, we will need to make a copy of your minecraft data on the cloud Dropbox server.
Windows
On Windows, your minecraft data is saved in your application data folder. You can access this by holding Windows+R or hitting Start > Run and then typing "%appdata%\.minecraft" in the box. This will open your minecraft folder. Copy all the files in this folder to your Dropbox. For organization sake, I usually create a folder inside my Dropbox entitled "Minecraft".
Once these files are done copying, navigate back up to your appdata folder, and rename the .minecraft folder to .minecraft2 This step ensures that you 1) Have a backup of your files in case of catastrophe, and 2) allows us to make the symbolic link to the correct location in a later step.
Mac OS X
In Mac OS X, the minecraft data folder is located in your Username > Library > Application Support folder. You can get here quickly by opening Finder > Go > Go To Folder > type ~/Library/Application Support/minecraft. For future reference, it's best to make an alias for this folder and copy it to your desktop.
Now, copy your entire minecraft folder into Dropbox. It is a good idea to rename the original minecraft folder (in your Application Support folder) to minecraft.original, to be saved as a backup in case something goes haywire.
Linux
In Linux, your minecraft data folder is located in your user home folder. However, it may be hidden from view. If you are using a GUI, ensure that you choose View > Show Hidden Files.
Copy the contents of the .minecraft folder to your Dropbox. As mentioned above, I usually create a folder inside the dropbox called "Minecraft".
Once the copy is complete, rename the .minecraft folder to .minecraft2
Alternate (command line) method
Open the Terminal and type in:
cp -r ~/.minecraft ~/Dropbox/Minecraft mv ~/.minecraft ~/.minecraft2
Create the Symbolic Links
This is the final step that will allow us to create links to the cloud versions of your files.
Windows
In Windows, the command mklink or junction will allow us to create a symbolic link. Symbolic links are similar to shortcuts, but are much more powerful.
'Note: If you are running Windows XP, you need to download Junction and place it in your root folder in order to enable symbolic links, which is available here.'
Click Start -> Run and type cmd in the box. Press enter. You'll be presented with the command line interface.
Type the following command to set up the symlink (be sure to replace <username> with your username!):
Windows 7 & Vista
mklink /d "C:\Users\<username>\AppData\Roaming\.minecraft" "C:\Users\<username>\Documents\My Dropbox\Minecraft"
If you get and error Cannot create a file when that file already exists. Try this...
mklink /d "C:\Users\<username>\Documents\My Dropbox\Minecraft" C:\Users\<username>\AppData\Roaming\.minecraft
Windows XP (after installing junction.exe)
Open the Terminal and type in:
junction "C:\Documents and Settings\<username>\Application Data\.minecraft" "C:\Documents and Settings\<username>\My Documents\My Dropbox\Minecraft"
If you are successful, the .minecraft folder should reappear in your appdata folder. When you start Minecraft after this, you should have all your previously generated worlds!
Mac OS X
Creating symbolic links in Mac OS X is easy thanks to a free application called SymbolicLinker, which will allow you to create a symbolic link via right-click in the Finder.
Download and install SymbolicLinker, available here: [1] (free).
Once installed, move your minecraft folder (located in ~/Library/Application Support/) to a folder in your Dropbox. Rightclick on this new folder and choose Make symbolic link. This will create a symbolic link in the same folder. Move this back to your minecraft folder's original destination (make sure it has the same name as the original) and you are all set to pull your game data off the cloud.
Alternate (command line) method
Open the Terminal and type in:
ln -s ~/Dropbox/minecraft ~/Library/Application\ Support/minecraft
Linux
To create the symbolic links in Linux, open up the terminal and type the following commands.
ln -s ~/Dropbox/Minecraft ~/.minecraft
Now, the .minecraft folder should be listed in your home directory and your saved worlds should all be there when you start Minecraft.
In order to link any additional computers you wish to play on, you just need to follow these steps - install Dropbox, and create the symbolic links. Voilà!
Note on Multiple Operating Systems
If you wish to sync your minecraft files between different operating systems, it is likely that the method above will not work. This is because the bin folder inside your minecraft folder, which contains binary files which run the program (as opposed to data files, like your saved games) may be different depending on your operating system. It is possible that Linux and MacOSX are compatible in this respect, and possibly Windows Vista and Windows 7, but certainly between Windows and either Mac or Linux you will have trouble.
If you have this problem, the solution is not difficult, just a little painstaking. When you have followed the instructions in section 2, you can delete bin from the minecraft folder in Dropbox. You could always leave it there, but you're not going to use it, so it takes up space. Then, you'll have to do a slight variation on section 3. Remember to make a copy of your original folder as per the instructions, in case something goes horribly wrong. Go into your minecraft folder, and delete all the files and folders except bin. Then you'll have to create individual links to each of the 5 folders and files other than bin (saves, resources, texturepacks, lastlogin and options.txt). You can make these using the same methods shown above. Note that on windows, though you should use mklink \d for saves, resources and texturepacks, because they are folders, you should just use mklink without the \d option for the two files, lastlogin and options.txt.