Openfire (Score 2, Interesting)

openfire is a jabber based FOSS server.
we use it with AD integration. I haven't implemented it yet, but they have plugins supporting full message transcript.

Spark is the client from the same company and it is jabber only.

If I remember correctly, openfire alos supports being a proxy for all other (most?) IM protocol's so even if someone gets a copy of AIM or whathave you on you network, there server can still log the transcript.

Easy to set up, free and robust.