Muck Rack, which aggregates tweets by professional journalists, instantly became my least favorite web startup when it launched in April with an endless stream of chatter from the Fourth Estate. It seemed to combine two characteristics that can plague both journalism and Twitter: self-absorption and the absence of a filter. I enjoy following some journalists on Twitter; following all of them is a punishment no one deserves.

But after several updates to the site — culminating in a refresh that went live this morning — Muck Rack has transformed from a cesspool of banality into a truly useful site probably worth a bookmark. Now users can sort journalists’ 140-character dispatches into broadly defined beats. Among the most interesting to watch are technology and arts and entertainment. (The most promising beat: weather, though now it only features ABC’s Sam Champion and a surprisingly good feed from USA Today.)

The other recently added feature worth grokking is Muck Rack’s list of trending topics, which this morning includes Gitmo, #Cheney, and Adam Lambert. (Why are journalists writing about the loser of “American Idol” rather than the winner? Remember, “comfort the afflicted.”) At right is a comparison of trending topics on Muck Rack and Twitter writ-large at 7 p.m. yesterday: You can’t say we aren’t a serious bunch. Muck Rack users can also view popular links among journalists on Twitter (incredibly useful) or browse their latest twitpics (fascinating).

Some of these new filters are similar to a project that Dave Winer is working on: Here, for example, you can follow tweets from employees of The New York Times, or perhaps you’d prefer to see what the 100 most popular people on Twitter are saying right now. Dave, who is a friend of the Lab, has called the effort “investigative journalism.” It is, at least, very interesting.

Muck Rack is in this for money, part of a small, New York startup called Sawhorse Media with several projects designed to capitalize on Twitter’s hype. They have similar verticals for tweets by, among others, musicians (great), venture capitalists (kill me), and pets (OK). Greg Galant, chief executive of Sawhorse, told me yesterday that the company is currently focused on launching a new vertical every week.

One obvious question raised by Muck Rack is who counts as a journalist. Galant said Sawhorse hasn’t committed to any particular definition but prioritized adding professional journalists from major publications. In general, I support an expansive definition of journalism, but for Muck Rack to have any use, it needs to be exclusionary in some way. They’ve received thousands of submissions for new journalists to include.

“It’s mostly people submitting themselves,” Galant said.

Another use of the site is simply keeping track of all the Twitter accounts at a particular news organization. (This has been done before in more exhaustive forms.) Galant told me that Business Week and USA Today have submitted exhaustive lists of their Twittering journalists, and an internal page at the Times points to Muck Rack’s collection of its people because the newspaper doesn’t have its own landing page for Twitter accounts at the Gray Lady.