I've been using Scribophile on and off over the past few years. The community is very friendly, and the critiquing features are top-notch. I've gotten lots of great feedback over time. The karma system makes it a little time-consuming to earn points but ultimately it's more than worth it.
I've been using the browser plugin for some time and it's a great way to catch a lot of common errors and improve your writing over time. The paid plan is a bit expensive but the browser addon is spot-on.