If there are any sites of interest which I'm missing/forgot, send me an e-mail and I'll add them.
Other reader-centric ponyfic reviewers
The City of Doors: Posts irregularly, averaging 1-3 posts a week. Reviews mostly MLP fanfiction, along with some original fiction and D&D-related material.
Problematic Conversations of an Overly-Expressive Unicorn-Loving Moron: About one review a week, along with occasional episode analysis or the like.
FOB Equestria's reviews page: A site for pony fans who're in the military, which (among other things) features occasional fanfic reviews. Focuses mostly on military-themed stories. Updates unpredictably.
IAH's Blog of Pony Stuff: Over a dozen fanfic reviews, plus various other analysis. Not currently active on the fanfic review front, but does continue to review canon work as it comes out.
Ezn is a Tumblr hipster: Exactly a dozen fanfic reviews, plus some great writing advice. Not currently active.
Jade Singer's Fanfic Reviews: Does detailed for-reader concept reviews of fanfics by request. Only a couple as of this posting, but more are expected.
presentperfect's FiMFiction blog: Posts 3-6 short reviews once or twice a week. Also has a chapter by chapter snarking of Fallout: Equestria which is pretty darn funny.
DPV's FiMFiction blog: Consisting of sporadic Weekend Fic Recommendations. Not currently active. Sticks to positive recommendations.
Bugs the Curm's FiMFiction blog: Massive posts once every one to two weeks, containing cartoon commentary, animation history, pony-related observations and/or griping, and yes, fanfiction reviews.
John Perry's FiMFiction blog: Posts reader-oriented group reviews of all the fics in the feature box once a week. Only a few posts at the moment, but more are expected.
Soge's FiMFiction blog: Posts short group reviews and ratings of whatever Soge has been reading recently, 1-3 times a month. Only a few posts at the moment, but more are expected.
Seattle's Angels on FiMFiction: Every week, three reviewers each review (the same) three relatively unheralded fanfics. Sticks to positive recommendations.
Places to find fanfics
Equestria Daily--If you've never heard of EqD, what rock have you been living under? Fanfiction here is vetted by the pre-readers, so the overall quality is pretty decent.
FiMFiction--The largest single repository of pony fanfiction.
FF.net's MLP page--Complaints about ff.net's userbase and layout notwithstanding, many excellent fics are posted here.
Pony Fiction Archive--Like FimFiction, this is an all-MLP fanfic site. It's quite a bit smaller, but with over a thousand stories, "smaller" is relative.
Pony Fiction Vault--An attempt to separate the fanfic wheat from the chaff, and bring some attention to deserving fics. No longer active, but still has over 100 stories (and attendant author interviews) in its archive.
The Royal Canterlot Library--A spiritual successor to the Vault, which posts high-quality stories along with author interviews once a week.
Places to get help for your fanfic
ponychan's and MLPchan's /fic/ boards--Great place to send a story that hasn't yet seen an editor/pre-reader. If you don't know where to go/what you're doing, toss it in the Training Grounds thread with a link and description.
The Equestrian Critics Society (on FiMFic)--A place where you can request a review of your fanfic. From their page: "We are firm. We are blunt. We do not sugar-coat. We are honest, unbiased, and fair."
The Proofreader Group (on FiMFic)--If you're on Fimfiction.net, this is one of the largest groups for pairing authors with editors, pre-readers, or other assistance. Read the rules, get help, repeat as necessary.
Struggling Authors (on FiMFic)--A large group for authors who want to get more exposure, or improve their story. Hint: the two generally go hand in hand.
Helpful stuff for aspiring ponyfic authors
EqD's Editor's Omnibus--Some of the stuff in here is solely for authors trying to get their works published on Equestria Daily, but most of it is great general advice.
Ezn's guide to writing (fan)fiction--Full of useful advice on grammar and style basics, and surprisingly readable.
Your local library--There are too many great books about how to/how not to write for me to list. Go check one out.