Google has opened its new Wave messaging platform to developers, as it promised to do at its developer conference last week.

Wave testers have embraced the service with enthusiasm. They're conducting experiments like embedding Waves in WordPress blogs -- not accessible unless you have a Wave sandbox account -- and contemplating ways that the platform can be extended.

"Just wanted to say great job to the team that created Wave," said Drew Burlingame, a Seattle-based software engineer who used to work at Microsoft, in a post on Google's Wave developer forum. "It is a great base on which to build solutions to a wide breadth of problems. I already have several apps in mind that were just waiting for a framework like this to build on."

Google also made test accounts available to a few technology journalists. What follows are some initial impressions.

Wave is a real-time communication and collaboration system that aims to be "what e-mail might look like if it were invented today." But that description overemphasizes Wave as a product and application. Wave is also a platform and protocol, and it is Wave's potential as open source collaborative plumbing for the Internet that makes it so interesting.

The Google Wave API is intended for two main types of development: the creation of extensions, programs that perform automated tasks by interacting with Waves or Wave participants, and the creation of embedded Waves, which are Waves that interact with other Web applications. There's also the Google Wave Federation Protocol, which allows Waves to be shared over the Internet between federated Wave servers.

Wave as an application is at once thrilling and frustrating. It's a bit like traveling back in time with a fax machine to a point when only a handful of others had fax machines. You know faxing will take off but you're stuck with postal document transfer until the fax network develops.

Wave also manages to be familiar and alien at the same time. It's familiar because using Wave is very similar to composing an instant message and writing an e-mail. But it's alien because it's not always clear which usage metaphor applies to what you're doing. I sometimes found myself wondering whether conversations being conducted in real time had shifted to asynchronous e-mail mode when a reply wasn't immediately forthcoming and it wasn't clear whether the other Wave participant was away from his keyboard or attending to a different application.

Lars Rasmussen, co-founder of the Wave team, acknowledges that there are a lot of subtle issues that have come up as a consequence of Wave's hybrid nature. He points to a simple difference between e-mail and IM apps: In e-mail and word processing programs, he says, the enter key moves the cursor to a new line. In an IM app, hitting the enter key sends the message.

"When we combine application functions," he said, "there's only a single enter key, so we have to choose if it is going to send or make a new line."