Dropbox is coming out of beta and will allow paid 50GB plans within the next week or two. I'm testing out the 50GB plan and am generally happy with the service.
Dropbox auto-syncs files in a specified local folder across Amazon S3 and your other designated computers. The clients are good on both Mac and Windows platforms. Versioning comes for free.
For small program and configuration file changes, the syncs are fast. Any changes to a file on my MacBook Pro are usually reflected on a Windows desktop within seconds. Larger files and volume syncing can take a long time, e.g., many hours to a day for 20GB. I'm not sure if this slowness is due to lots of testers all trying to upload the extra 48GB quota at the same time.
If you sign-up, pay attention to the dropbox cache folder. This folder can grow pretty large (GBs) due to active downloads. After downloads, the folder will be auto-purged, but during large syncs it can get large.