A lot, but not enough. In place editing of HTML, CSS and cookie manipulation are almost non-existent, especially compared to the way firebug allows it.
Have you actually tried WebKit's developer tools lately?
It has a great DOM inspector (allows editing), network/script/rendering timeline, step-through debugger, profiler, cookie and local database inspector, and Chrome even has a heap analysis tool (see which objects are allocated).
It's far better than Firebug, at least for JavaScript developers, IMO.
The element inspector comes somewhat close, but you can't add css attributes to selectors, only modify existing ones, which really makes the thing useless when doing front end dev
unless you're seeing something I'm not seeing, you can only modify existing attributes. If you could show me though, an upvote will be in your future :-)
Though not as useful as firebug you are able to add a 'style="<custom css>"' attribute to an element by hovering over and clicking the '?=""' that appears.