off-the-record messaging is a standalone library which has also been included in a gaim plugin - its encryption with NO digital signing at all, however it still provides authentication (unlike trillians secureIM, which doesnt let you know WHO the hell you're talking to, despite the encryption.) it works for both nix and windows versions of gaim - i'm no encryption expert, but i sure um...feel a lot safer when using it.

perhaps someone more qualified than i can peruse their whitepaper and give some more informed feedback as to the security/robustness of the protocol/implementation :)