Major outlets to get online TV shows, whether present, or previous seasons:

legit

• Hulu [free] [Linux]
  • Hulu Plus [$]
• Amazon Instant Video [w] [$, some content is free with a Amazon Prime subscription] [Linux]
• previous seasons only
  • Netflix [$] [no Linux]
  • Blockbuster On Demand [$]
  • iTunes Store [w] [$] [no Linux]
• more at Wikipedia
• more specific at Wikipedia

possibly/probably illegal

• note: there are A LOT of scam sites that suggest they offer way more than they really do
• sidereel.com — a multi-site index, for each episode
• EZTV
• more at scamfreeinternet.com
• lots of torrent sites