Are anime streaming sites legal?
There are a number of legal streaming sites: Crunchyroll, Funimation, Manga Entertainment, Daisuki (which is relatively new), and some Youtube channels associated with legal sites (such as Funimation or Manga Entertainment), possibly among some other sites that I haven't heard of yet or am not completely sure about. Some of these are very region-specific (e.g. a lot of Funimation videos on Youtube can't be accessed outside of North America (I think)).

Most other streaming sites (including some pretty popular ones, it seems) are actually not legal (using, as stated by @Oded, rips of videos from elsewhere, illegal fansubs, etc.).