I don't remember who first showed me these, but there are several websites that I have found valuable.
Overwatch Fire : http://www.owfire.com/overwatch/counters
Has a good lineup of community voted counters to each hero. The site also hosts guides submitted by users for team comps or heros.
Master Overwatch : http://masteroverwatch.com
Primarily a profile stat tracker that will give you information on each hero you use with a bit more detail and increased accessibility than with the in game profile stats.
News & Discussion r/Overwatch : https://ow-01.reddit.com/r/Overwatch
Probably already well known, but still a decent place to discus anything related to overwatch. Unfortunately it is mostly flooded by suggestions.
Personally, I think the Blizzard forum is a mess, but here it is anyway: http://us.battle.net/forums/en/overwatch/
If you have any other sites or resources, feel free to share.
Another stat tracker. Similar to Master Overwatch.