If you follow some of the technology news site you might realize how frequently Google brings out some new thing or else. I don't claim that their ideas are all orignal but they seem to display them the first in most cases. This all simply shows that Google is already the leader (not in revenue of course but) in (re)innovation.
The case is something like: if google is using some technology, everyone else will start thinking of using it at the least some such things which google made famous may include:
WebSearch(yes, it was almost dead 3-4 years ago and see it now)
Image Search.Video Search (Yahoo, MS both seem to have these things in their portals now)
Maps (MS is aggressively doing research & Development(:-) ) on maps now.
AJAX (the newest thing made popular by google interfaces)
Ad sense( See yahoo starting something similar with name wordPress)
Blogs (see yahoo 360, MSN channel 9)
Orkut (see yahoo 360)
Piccassa and Hello (see yahoo 360, yahoo buying flickr)
And last but very important Webmail innovation. (1 GB was a dream earlier, now it is minimal)
I came accross this site which shows that Yahoo is now doing almost everything that google does. See the list of project you will understand.