Gary Thorne is the TV play-by-play man for the Baltimore Orioles as well as being with ESPN. Tom Paciorek was with the Florida Marlins last year, but I don't think he's with them anymore.
As far as Jack Brickhouse's broadcast partners on WGN-TV (back when Channel 9 covered both sides of town with the same guys): Vince Lloyd and Lloyd Pettit both died in 2003. Can't find any data about Harry Creighton, but given the fact that he was with WGN in the late '40s through the mid '50s, I'm assuming he's no longer with us.
Moving over to the north side:
DeWayne Staats is now the voice of the Tampa Bay Devil Rays.
Josh Lewin turns up on the Fox Game of the Week on occasion, but I don't think he's with any team at the moment.
This year, Thom Brennaman joined his dad in Cincinnati after 9 years in Arizona.
Jack Quinlan died in a car crash in Arizona in 1965. Lou Boudreau died in 2001.
They also had some guy named Reagan recreating games from ticker-tape for listeners in Iowa in the '30s. Wonder what ever became of him? ;D
Chip Caray, Steve Stone, and Bob Brenly will be part of the playoff picture on TBS this year. Caray and Stone will not be paired up, unfortunately.
The original baseball broadcaster (both Cubs and Sox) was Daily News sportswriter Hal Totten on WMAQ (owned by the Daily News at the time). He began broadcasting in 1924. In those days both teams allowed any station who wanted to do so broadcast the games. The Sox didn't award exclusive broadcast rights until 1944. Not sure about the Cubs.
Link: Wikipedia list of Sox broadcasters