If you weren't aware, AIM 5.2 and above allows for secure encryption using an X.509 certificate. You can either get an untrusted cert for free from AIM Encrypt or get a trusted one from Thawte or CA Cert for free or buy one from Verisign. The problem with securing IM's is that both parties must have X.509 certificates for it to work.
The same can be said about Trillian. Trillian can only secure IM's with other Trillian clients. Trillian's Secure IM protocol is only semi-secure as it is vulnerable to spoofing or "man in the middle" attacks during its Diffie Hellman key exchange.
To view other alternative messenger services, you can view this article:
**broken link removed**
Here is more information about encryption on AIM:
AOL Instant Messenger Encryption
The latest versions of AIM now have the ability to seamlessly use your email certificate to encrypt your AIM conversations with any other Instant Messenger user who is also "Security Enabled." This is easy to tell, as the "Security Enabled" users have a lock icon next to their screen names in your buddy list.
To set up your AIM to use encryption, export the full path of your certificate from the IE certificate configuration window, including both the public and the private keys. You should then be able to import the certificates into AIM under My Preferences -> Security -> Advanced. Besides having to type an extra password to give AIM access to your private key when you log on, using encrypted AIM is just as easy as it ever was!