If you want to watch the full series, DVDs are the only option. Dailymotion has some of the episodes, but it doesn't have any. And, even so, the DVDs contain some scenes that are missing due to music licensing issues, as mentioned. But they have each and every episode.
The group R%26B Color Me Badd had a significant presence in 90210, but online sources report that negotiating music rights got in the way of broadcasting episodes. I think they even cut some scenes within episodes, I was watching Season 4 the other day and they cut out a scene that I remember had music. Finally, season 10, which is the last season of the series, has episodes 2, 6, 10, 12, 14, 16, 18, 20, 24 and 27 missing.