The chat interface surely is the most important part of a chat program.
There are several solutions, based on two central plugin architectures; SRMM (Send and Recieve Messaging Module) and nConvers.
SRMM is the standard messaging module supplied with Miranda. It is simple, and straight-forward, and although lacks some features, is perfect for out-of-the-box use.
tabSRMM is based on SRMM, but adds tabs, and (now) a whole host of new features. Probably recommended for new users who wish to add more features to SRMM.