Hundreds of scripts, for a wide variety of popular sites, are already available at http://userscripts.org.
You can write your own scripts, too. Check out http://wiki.greasespot.net/ to get started.
The Twitter Search results on Google userscript places real-time results from Twitter Search into your Google Search results. Kind of like what the Feedly and the StumbleUpon extensions do. Check out the screenshot below to get a better idea.
If you’re interested in other GreaseMonkey userscripts, check out Import Your Twitter Friends Into FriendFeed.