depends on which one. lol.
there are tons of them out there. i used to do that, but the servers always went down because of bandwidth limits.
i even have a copy of a version of the private server and set up my own little WOW lan party for a while, then all the school work came and i had to take it of because i lacked hdd space.
they aren't as good as the original, but i think one of them did go as far as having proper instances and working out a lot of the bugs, this was right before they started the battlegrounds thing. after that i lost interest and the site i got the server from went down.