- The other one (NetMessenger?)
The first one is free, and is a native client -- it connects directly to AIM's Oscar servers. Therefore, the client is free (though, as SRA notes, not Open Source), and there is no annual service fee.
The latter two are both proxied clients -- they talk to a central server somewhere, which then in turn keeps your session up with whichever chat network you want to talk to (Ramble is AIM only).
Because they have to put the proxy *somewhere*, and pay for it's connection, both are commercial products, and charge an annual fee.
The fee isn't that bad, honestly; I'm trying out Verichat and Ramble right now, to see if it's worth it (since I'm an AIM-only guy).