I recently received my invite to the Google Wave preview! It's better than I thought!
Google Wave Website
After watching the hour long demo video that Google gave at some conference, I was worried that this application would be cluttered with so many features that you would struggle to do basic every-day tasks like sending a message to somebody. Having finally tried it for myself, I now know that I was completely wrong!
Nothing needs to be downloaded to use this application. That fact alone is enough to make me prefer this over all other messaging and collaboration apps. Now that I'm registered with it, I simply go to wave.google.com, and I'm instantly there.
A "Wave" is an online object that can consist of any kind of media you can image. Some examples are: text, images, links, videos, google maps, a voting box, and any other 'gadgets' that developers will create in the future.
Any one of your contacts that you add to a wave can participate in it, meaning they can view, contribute, and comment on anything there. Think of it like an informal wikipedia article within a small circle of people.
The design of this program is clearly influenced by Facebook and AOL Instant Messager. Users have an avatar/buddy icon that represents them. You can set your away message/status. Any media added to a wave can be 'commented' on. You are informed which of your contacts are online and offline. You can organize contacts and waves into 'folders'. Waves can be 'tagged' so that you can easily search and categorize them. Clearly Google took popular internet communication conventions and put them all together in one application
If enough people start using this instead of other messaging apps, Google Wave could easily replace them all