I got this idea when I was thinking about using Sugarsync for my music library so I have the same music on my laptop and my PC and realized it would work for the Minecraft worlds as well. In my opinion this is how minecraft should be played. This is a tutorial on how to set it up to work for yourself and you could be punching trees in no time.
Step 1. Have Minecraft installed on our original host and your other computers. For me this was my desktop pc and my laptop. Download and run it if you have not already done so on both machines. I am assuming everyone knows where to find Minecraft and how to run it.
Step 2. Download and install Sugarsync on both computers. You can get it here. You get 5 GB free storage which is more than enough room. Go through all of the steps and stuff.
Step 3. Click on the tab on the top left corner that says Add Sync Folders.
Step 4. From there a window will pop up. Click the little arrow on the side of your C: drive then Users next the name of the account you are logged in on (mine happens to be my name) then AppData and finally Roaming. Mark the little check box for your .minecraft folder if you want everything to be the same like texture packs and stats or mark just the Saves folder if you want just the worlds to transfer over. If the .minecraft folder is not there than you need to run Minecraft and try again.
Step 5. We are done with your host computer for the time being. On your other computer go to the start menu and in the search box type in %appdata% and open your .minecraft folder and delete everything in there or place your Saves folder on your desktop for safe keeping if you want to save your worlds on this computer. If you don't have a .minecraft folder run Minecraft and then try again.
Step 6. Run Sugarsync and click on Manage Sync Folders.
Step 7. Grab the .minecraft folder and drag it over the other computer and press the OK down at the bottom.
Step 8. This is probably the longest you have to wait during the whole setup but you have to wait until the files are done syncing before Minecraft will work. You will know when they are done syncing when all of the dots next to them are green and not yellow anymore.
Step 8. Now it is time to play Minecraft.
If you don't explore too much or install large texture packs it updates pretty quick. I have not messed with leaving other files out because all of them are smaller than your Saves folder which is the important one but I would leave the Bin folder checked because if you update it when you play it on one computer it will be updated the next time you play it on your other computer already. If you go to tools and then preferences make sure that Sugarsync is set to starting up when your computer starts up. If you just use Sugarsync for Minecraft I suggest setting your transfer rate to fast so you don't have to worry about your computer turning off before the files are synced. Just for fun i played minecraft on the two computers at the same time and if you change anything on one the other computer throws a Level Save Conflict.