depending on your niche some options are really great:
For visual sites (sites with lots of images) DeviantArt, Pinterest, Imgur, etc are great
For personal sites Reddit and Quora can work when done right
For business sites... well, businesses can be promoted everywhere
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
For gaming sites Youtube I think it's best
For local services sites (wedding planning, plumbing, etc) advertise in local newspapers, magazines, radio, post to local forums...
For content sites / blogs SEO is obviously the number 1 option, but social media also works (and so do some of the other options I've mentioned above, like Reddit, Pinterest, social media, etc)
It really depends on what type of sites you hav