Mac: The official Google Drive app for Mac isn't bad, but it's not that user-friendly. If you're looking for something a little more simple that fits in with the overall Mac design scheme, Archy is a new Mac app currently in beta that allows you to manage your Google Drive in a simple way.
Much like the official Google Drive app, Archy doesn't allow you to edit or create documents. It does, however, feature easy-to-use sharing and organisation. If you do want to edit, you can set up Archy to open files in any app you prefer (including offline Google Docs) and get notifications in Notification Centre when changes are made.
Considering Archy is still in beta, it has a lot of features, and we haven't had any problems running it on Mountain Lion. Archy is a free download from the developer's site below.