If you have the new windows vista, just go to the windows media center. all three seasons are on there for free. Just go into Internet Tv, go to tv, Under tv go to series, and you will see all three seasons available for free. The shows only have two 30 second commercials in between.
You can also go to hulu.com, which has all of season 1 and 2 available for streaming, with only a few commercials in between. On October 9th, 2009, season 2 will no longer be showing on hulu.com, but season 1 will still be available.
TVShack.net has all the seasons, without commercials, but they are missing a few episodes (9-11 I believe) of Season 3.
All episodes can also be found at the following link: http://arresteddevelopment.msn.com/
Hulu airs each season in rotation. I believe it is currently airing seaons 3 (2/3/2011)
If you have a Netflix Instant Streaming account you are able to watch all four seasons online on a Netflix supported device such as Windows, the Play Station 3, Xbox 360, or an Nintendo Wii.