LouisBlack covered the basics: iTunes, iOS devices, and Airplay. Here is my current setup:
Living Room: Mac Mini connected to 5.1 surround receiver/TV
Bedroom: ATV 1 connected to TV
Office: Airport Express connected to stereo receiver/speakers
Macbook and iPhone 4 are used throughout the condo. My movie/tv show library lives in iTunes on the Mini, although I primarily use Plex for playback on the TV. My music library lives on the Macbook, since I want access to that when I travel and use it to sync my iPhone. From iTunes on both the Macbook and Mini, I can select to play audio in all three rooms at the same time (Note: I added the recent Shairport hack to my Mini so it can act as an Airplay device). Using Airplay from the iPod app on my iPhone, I can stream to any one room, except the bedroom (because Apple didn't update the original ATV for Airplay). Using the Remote app on the iPhone, I can stream the iTunes library on the Macbook to all three rooms at the same time.
There are some newer Airplay speakers including the B&W Zeppelin Air and iHome iW1 that do not need to be attached to an Airport Express/receiver. Sonos also just released an update so you can attach an AE to it. This site seems to be tracking new developments as they come up:
http://airplayspeakers.com/