Using a service like Dropbox as a convient way to synchronize your game files between two computers (such as a desktop and a laptop) is really pretty easy. Here is how to create a directory junction in Windows from your game's normal save game directory and a special directory created in your Dropbox folder. I suppose this will work for other file synchronization services such as Ubuntu One, SugarSync, or Wuala also.
In this example I am using the game Terraria. I am going to sync both my saved worlds and players data in my Dropbox folder instead of the normal folder set by Terraria. This will work for pretty much any game, though. I use this for my special single-player Minecraft map also (seed: gargamel), so I can have it wherever I have Dropbox.
1. Determine where your save game folder is.
In this case, Terraria saves its user data in the directory:
This is folder we will be working in for the most part. There are two directories in here, Worlds and Players. These are the two directories I want to relocate to my Dropbox folder. USERNAME should be replaced with your actual user name.
2. Recreate the directories in your Dropbox folder.
To do this, I just copied the "My Games\Terraria" folder to my Dropbox folder. Now I have the following folder:
This contains all the saved data for Terraria I want to keep synchronized between my two computers. The location of your Dropbox folder may be different than mine, depending on where you installed it.
3. Link the folders.
Now I go back to the My Games directory and delete the Terraria folder. Horreur! I know, but remember you still have the copy in your Dropbox folder just in case. What we now want to do is load up Windows' handy-dandy command prompt (ie, cmd.exe), and using it we navigate our way to the My Games folder with the following command:
cd C:\Users\USERNAME\Documents\My Games
Remember, USERNAME should be replaced with your actual user name.
Now we create the directory junction to our Terraria folder in our Dropbox directory. In the Command Prompt, use the following command:
mklink /J Terraria C:\Users\USERNAME\Dropbox\Terraria
This will now make a "shortcut" to the Terraria folder in your Dropbox directory in such a way as to be indistinguishable from an actual normal folder to your system. You can play the game normally, and when you save it will use the "Dropbox\Terraria" folder as if it were the normal "My Games\Terraria" folder.
That is all there is to it. Repeat this process on your other computer, taking care not to overwrite your Dropbox-saved data with old information. Now, you will have access to the same saved game information regardless on which of your computers you choose to play it on.