
Chrome
If Chrome takes off, it will be because MS forgot to participate in the JavaScript performance war and they lose the developers. It'll be a sad day when IE on Windows run slower than MobileSafari on an iPhone for most JS pages.
The 30x-60x speedup of the JS engines in the last half-year makes things that were unreasonably slow before doable on the web now. That kind of performance increase opens up new app possibilities.
Safari and Chrome and Firefox are all in this hunt, and Opera is still several times faster than IE, but several of the new JS engines are open source, so MS could copy them quickly if they realize they've been caught flat-footed. The next major release of all the browsers will be interesting.