Google Wave is one of the newest ways to communicate over the interwebs, using a blend of email, wiki, and IRC technologies. In short, a wave is a document that multiple people can edit simultaneously. Nor is it limited to text; waves can support multimedia including embedded apps called "widgets". If someone should create a IF interpreter for Google Wave that lets players team up to play a game of interactive fiction, please let us know!
Google Wave is still in beta and a somewhat rough around the edges. (For example, the scrollbars are inexplicably non-standard and awkward.) At present, you'll need a Google account and an invite from a current Google Wave user to be able to use Google Wave. If you don't have an invite and want one, ask around for one. (As I type this, I have 25 invites just waiting to be given away. All I need are email addresses to send them to.)
By the way, at present, all waves are Google Waves, but that won't stay true forever. The wave technology is open, and other wave providers are expected to make an appearance on the 'net stage in due time.