http://imgur.com/gallery/a3cHR - Map callouts for the most played competitive maps.
http://www.thewarmup.net - A site where you can easily find videos of smokes on the popular maps.
http://www.faceit.com - And external league/ranking system with prices, have some premium features which costs money.
http://www.challengeme.gg - 1v1, 2v2, 3v3, 4v4 & 5v5 fights. Raffles of hardware/skins on a weekly/monthly basis.
http://www.reddit.com/r/GlobalOffensive - Primary CS GO subreddit.
http://www.reddit.com/r/CSGOBetting - Subreddit dedicated to info, analysis etc. of professional fights