No it won't run Crysis, but damn if the Avengina Project is not impressive. This Java-based graphics engine harnesses both the power of your Internet connection and your graphics card to run incredibly detailed 3D environments right inside the confines of your browser. It integrates lighting and graphics filters that can scale up depending on the hardware quality of your system.
Avengina's project page lets you take the engine for a run on your machine as long as you have a recent version of Java installed and meet the minimum hardware requirements. I found it to chug a little bit on my laptop, but that's only because it has a pretty ragged graphics card. Users with gaming rigs should have it running as smooth as silk--or at least as well as the demo video on this page.
One of the things that makes it stand out among other browser-based graphics engines is that it handles lighting particularly well. You get things like halos around light bulbs, and shadows that move depending on where the light source is coming from--it's very pretty, and the kind of stuff we've seen in install-based PC games for the last 10-15 years.
Daniel Seifert, the creator and sole developer for the Avengina Project, insists it's not being built for gaming purposes, but instead for "presentations." As the demo shows off, you can cram a lot of text and billboards within one of these virtual worlds, but sorely missing was anything to shoot at, or platforms to jump on--two things that make first-person environments immensely fun to be in.