Try
aMSN. It also has a lot of skins. Also, the success rate of file transfers is MUCH higher in aMSN. Plus, there are some nifty features, like the option to see all of your contacts by their e-mail address instead of their screenname. Also, you can have longer screennames!
However, I feel the disadvantages to normal MSN outweigh the advantages. A big disadvantage is that it uses a much greater percentage of the CPU, and thus it is a much slower program. Plus, pressing COMMAND+A doesn't always highlight the text, like when you're entering in a screenname (which has to be done like MSN Messenger for OS 9, instead of the more convenient way in MSN Messenger 4). Also, while it doesn't sign you out as often as MSN, it does sign you out when the computer wakes up from sleep mode. Plus, it can't record your history!
Now, there's also dMSN, A.K.A.
Mercury Messenger. I don't use it, so I don't know much about it, but I know it's kinda like aMSN, but a bit different.
And there's also
Fire. I know even less about it.