Rendezvous/Bonjour for Miranda: to send instant messages from a PC to a Mac (with Adium or iChat), or from PC to PC, without connecting to a remote ICQ or MSN server. Available as a Miranda plug-in or a stand-alone app (dubbed “Windows iChat”, based on Miranda, and expectedly working any need for configuration). It doesn’t seem to handle file transfers, but it’s still simpler than creating an MSN account per machine.

For now, you appear on the network with your Windows username, but the developer announces the possibility to change names for a future version (but then, he hasn’t touched it since April).