GMail Drive takes the short stack of gigabytes given to Gmail users and turns them into a virtual drive accessible from your computer's file tree.
In short, it creates a shell name--a space extension that creates a virtual file system around the account. This lets you treat those 5 gigs (at the time of writing) as a standard remote drive. You can drag and drop files into the drive, or use standard cutting and pasting. When you add a file, it sends an e-mail to your Gmail account with the file as an attachment. If you delete a file from the virtual drive, it deletes the associated e-mail, as well.
There's not much else users can do with GMail Drive. It's an effective tool, but lacks any additional features. It does allow for proxy authentication and secure HTTP, it preserves long filenames, and it can auto-login, a useful touch for those who want to regularly use their Google account for file transfers. Also, it'd be helpful if the install file was a self-extracting EXE instead of a ZIP. Although it's nice to have a virtual drive option that doesn't require using your Web browser, GMail Drive would benefit by adopting some of the features of its competition.