Place your brain in a dropbox folder and it will auotmatically replicate it with your other PCs and Macs and even leave you a backup on the web you can alos access. But the free dropbox is only about 2.5 GB, so if you brain gets very big you might need to upgrade dropbox to a larger capacity. http://www.dropbox.com/ A mac user should be able to use Mobileme to keep files synced.
I see Matt recommends against this. see the thread at http://www.thebrain.com/#-102 Sounds like sync problems occur. My experience was simple test files.
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