There's Adobe Flex. There's Microsoft's Silverlight. There's Java's JavaFX. Maybe there are others.
Cons: plug-ins. Not standard. Different user interface and different rules for every platform. Compatibility issues with browsers, operating systems, elevation of rights (admin rights for install, elevated rights for execution).
Most modern web apps (gmail, youtube) exploit the browser to it's limits, my firefox just can't keep up (here and here).
Their presentation of their new browser, called Chrome, is very ambitious:
If they can do what they promise, I'm going to use it, I promise.
They hit exactly where current browsers fail (or at least with me) - speed, saparation of different pages (for some time I've been wanting to create multiple firefox processes, but don't know how), separation of plug-ins (to know who's fault it is), sandboxes, minimized rights, incognito mode.
I don't know much about Google Gears, but for offline reading of my feeds, it dissapointed me all the times I tried to do it.