I second the recommendation of Insync. I use it on three Macs, and so far it has worked flawlessly as far as I can tell. I haven't beat on it mercilessly, but I did use it for all the course material for the two free Stanford classes I took, along with the Octave code I wrote.

Re how they are going to make money: They used to charge $20 a year if you wanted to sync more than the free 1GB that Google Docs gives you. This apparently was very confusing for customers, so now they will sync any amount for free and plan to offer some additional extra-cost features, but they haven't announced these yet. So at the moment, Insync is completely free.

I've lobbied them to support opaque syncing of symbolic links and to have a mechanism specifying files to ignore (a la .gitignore) as an extra-cost feature, so that I can use Insync for syncing code that I am working on between my work computer and my home computer. (I currently use Unison for this purpose, but I find Unison to be something of a chore to use.) Whether they will ultimately support these features, I don't know, but they did show some interest. At the moment, neither Dropbox nor Windows Live Mesh nor Insync are no good for this purpose, as none of them support ignores and both Dropbox and Insync treats symlinks transparently rather than opaquely. Live Mesh is a bit better than Dropbox and Insync with regard to symlinks: it just ignores them completely. This means that I could conceivably sync them out of band, but since it doesn't support ignores, the issue is moot.