Pidgin of course as it supports all of the protocols and it is cross-platform application. I use it time after time with AIM and G-mail protocols. IM sort of a must tool nowadays as it is way faster then email and helps to keep live conversations. It can be also good time waster machine if used improperly. About Skype, it is a great thing for voice conversations but Skype IM protocol kinda sucks as on some slow networks (like wifi AP)it is unable to deliver messages sometimes for dozens of minutes. Same about MSN, protocol sucks big time, not even works on some remote networks. AIM keeps malfunctioning weekly but if you don't use MSN or Yahoo and you work with US companies it is a must. G would be the winner but it still rarely used by others, besides I have privacy concerns with it, but this is I guess current situation with any other IM as well.