You can set up a friends list, and turn on chat only for that friends list, if you like. Set up the friends list at https://www.facebook.com/friends/edit/ and then click the little settings icon at the bottom right (in the new chat design) and choose "Limit availability...".
With the old chat design, choose chat at the bottom right, then the "Friend lists" menu item in the chat window, and add the friends list you want to affect chat availability, and then turn availability on/off to particular lists.
Useful for parents and work colleagues who don't use other IM or who you don't want to give your personal IM accounts to.
I think this discussion made me realize that I want there to be a minimum barrier to contacting me in a way that requires instant feedback. I don't care who has my e-mail address because I can reply to e-mail on my own time and terms, but I'd rather people have to do a little work to contact me via IM, and I want it to be pretty hard for people to casually call me just to chat.
With the old chat design, choose chat at the bottom right, then the "Friend lists" menu item in the chat window, and add the friends list you want to affect chat availability, and then turn availability on/off to particular lists.
Useful for parents and work colleagues who don't use other IM or who you don't want to give your personal IM accounts to.