Twitter reports on their blog that they acquired Summize, a search engine that indexes twitter posts.
As Twitter co-founder Biz Stone puts it:

“The folks over at Summize discovered Twitter and significantly improved on this idea of filtering and search..”

Techcrunch reports that Twitter hired 5 of the 6 employees, all of them are engineers. The original founder and CEO moves along to a new project. The Summize API will be merged with that of Twitter and integrated under the Twitter brand. Twitter is expected to add search and related features in the near future.

Nobody mentioned numbers, but Silicon Alley Insider reports a total of $15 million, both in stock and cash.