iTalk doesn't yet support conference calling, or encrypted calling, and doesn't have the Skype missed calls list, but other than that appears to be a functional clone of Skype. It's also a bit more standards based, in that it supports H.323, rather than a proprietary signalling protocol, but it uses a proprietary codec, which makes that somewhat moot. Voice quality is similar to Skype, which is to say very good, but I don't know how well it will handle dial-up. The codec seems to be some kind of a learning codec, in that quality improved over the duration of the call. MSN messenger is the same way.
Click on over to Alec's blog for the rest of his rundown and screenshots.