Dropbox (which I also highly recommend), creating a folder on your machine. This allows you to drag/drop files that you want to upload to Google docs. It will not automatically convert them, however. You will need to use the Google Docs interface for that.
One cool thing is that this application integrates with Growl (a notifications application). Any time a document is updated (any shared docs) a notification pops up unobtrusively onto your screen. This is a nice way to be aware of changes to documents.