As a recent communique to its beta testers states, Vastpark, the other virtual world development platform other than Metaplace, will enter open beta in April. As such, it's calling on its testers to provide a stress test. Now, unlike Metaplace, Vastpark requires that its creators download proprietary software to make everything work, but it plans on releasing an SDK for full user customization. Currently the software is PC-only, but a Mac version is planned.
Their website shows an impressive array of screenshots and video -- when I lamented the state of what Raph Koster was showing at GDC this year, this is the sort of thing I was envisioning. Vastpark is the next logical step in the evolution of virtual worlds that Second Life pioneered. Time will tell whether it garners the same community and devotion.