In your own space, promote three communities, challenges, blogs, pages, Twitters, Tumblrs or platforms and explain why you love them. Leave a comment in this post saying you did it. Include a link to your post if you feel comfortable doing so.
Because of my OTP love, my first rec is for the home of all things Snarry: snape_potter. They exist for people to post stories and art, of course, but people also can post meta, questions, searches... Plus, they host two of my favorite fests, the Snarry-A-Thon which posts around Easter, and the Snarry Swap, which just finished posting. And they're incredibly organized! They have the entire year planned out ahead of time, whereas I can't even plan what's for breakfast tomorrow. :P
If your taste runs to kink, then I present: daily_deviant.
The site had been in a bit of a slump for a while, but seems to be reviving with the addition of new members. Anyway, I'm pretty sure you can find just about any ~unusual kink there, just search the tags. Disclaimer: yes, I'm a member, so this could be seen as a tad self-promotional. *shrugs*
Finally, I present one of my very favorite fests of the year, bar none: hoggywartyxmas. This fest is chock full of rarepair and characters studies, and I don't know how the mod manages it, but the creations are always stellar. I always rec a ton of art and fic from it and often times have trouble not just reccing the whole thing and calling it good.
And, as a bonus (and because I always do this, so why quit now?), I'd like to pimp the drabble communities, i.e. snarry100, snape100, neville100, hp_nextgen100, harry100, dracoharry100, mixandmatch100, femslash100, just to name a few. I think drabbling is an underrated writing activity. IMO, it helps you hone people's writing skills. Give it a try!