Technorati, a service that indexes blogs in real-time and provides search, indexing, and link-analysis, has added "keyword tracking."
For example, say you're interested in keeping track of the recent rumor that Six Apart is buying LiveJournal. You would start by going to Technorati and typing in a set of search terms like:
("six apart" OR sixapart) AND (livejournal OR "live journal")
This will give you an instantly updated stream of posts from blogs around the world that are talking about both SixApart and LiveJournal, in a post, using a variety of spellings.
Note the results page, however – Underneath the title of the search, you'll notice a link that says, "Make this a Watchlist". Click on that link, go through the login process (or create an account if it is the first time at Technorati), and you'll get a link to that saved search to put into your favorite RSS reader.
Linkvia Sifry's Alerts)
(Disclaimer: I'm an advisor to Technorati -Cory)
