Ages ago, I added a button that allowed people to mark users as friends. It didn't do a whole lot up until now.
Now, you can see your list of friends and the list of people who have friended you on the Friends and Followers tabs on your user profile, respectively. There's also a new block on the home page that shows the latest art by your friends. You can click on the block title to expand it into a full page view.
Note: Friend lists are currently public. If this is a major issue for anyone, please let me know.