Okay one of my favorite things that the Organization for Transformative Works is their fanfiction archive, Archive of Our Own. I literally go to that website everyday. Founded in 2007, Archive of Our Own (more commonly referred to as AO3) is now the third largest archive for fanfiction.
For the fiction that is available on the site already, there is the most comprehensive and easy to use search engines on any fanfic archive site. You can search by character, ship, language, rating. Rank my comments, kudos, newness. These are only a few of the features you can use to search. I've been reading fanfiction for over ten years now and this is the best search engine I've ever come across. Even better is the tagging system. They make it easy to either use already popular tags that will help people better find things related to what they want to read or the author can make up their own tags so they can be a specific as they want.
One of my personal favorite features of the site is the ability to add a piece to your account without posting it yet. The site will keep your work for up to a month. This is great for writers who have finished something and may want to keep it for a little while before posting it. Because many fanfic writer participate in bangs or exchanges, it is nice to be able to semi-post something while waiting on the deadline.
The volunteers for the website also run Twitter and Tumblr accounts to keep their users updated with anything that is potentially going on with the site. This is great for times when the website might be down for maintenance or there are problems with your account and you need someone to contact.
I could easily go on and on for pages about the wonders that AO3 brings to the universe. They are a marvel for both fic writers and readers. Despite there being a myriad of archives out there, this is the main one I use. Partly because it is gaining so much popularity, but also because it is most conscientious and helpful websites out there.