A news aggregator is a bit of software that helps you keep track of news. Most people use them to follow ridiculous numbers of WebLogs
A comprehensive list of news aggregators is available at http://www.hebig.org/blogs/archives/main/000877.php. Some included there are:
Windows / Mac
http://radio.userland.com/whatIsANewsAggregator - an introduction to news aggregators by DaveWiner?, with a focus on the aggregator built in to his own RadioUserLand.
See also MeatBall:ChangeAggregator