Pueblo, in addition to being its own client, is a special MU* protocol which allows games to send hyperlinks, images and sounds, among other things, to the client. Pretty much, Pueblo was an attempt at making MU*'s more graphical and mouse-friendly. Pueblo is NOT to be confused with MXP, which is ZMud's client-side attempt at the same function with a more popular, but different language.
Thus far, the only known fully-enabled pueblo client is its namesake: Pueblo. However, MUSHClient includes many of the features of Pueblo, although a few of them have been either disabled or dumbed down in order to retain the feel of a purists MU*. ZMud claims to have Pueblo emulation, but for the sake of the Fairy Garden, this has been proven to not work at all.