Rated 5 out of 5 stars
See KallOut (www.kallout.com)
I agree with you that the killer feature in IE-8 is selection-based search (aka Accelerators or Activities) where you can get content with only your mouse. This is a neat firefox addition.
A version of selection-based search which I LOVE is KallOut at www.kallout.com.
Kallout gives results in context over any website or Microsoft Office (not just IE8). Kallout already works over IE and Firefox as well as all of office. It’s really a killer application IMO for selection-based search.
The problem with Accelerators is that the user has to figure out which one to apply to his selection. It’s just unusable once you get more than two or three. What accelerators really needs is a way for the application to automatically choose which accelerator you need to use for your selection-based search request. KO picks the right accelerator based on your selection with BestGuess. Just a random reader opinion, anyway.
Check out Kallout at www.kallout.com for yourself!