Only trustworthy way is build one your own or have someone trustworthy write it up. It's very simple. Just a TCP connection, an IP address broadcaster and a gui.

I'd never trust a random plucked from the net IM. Even the biggies are not secure. 

Been using this for years...works great! http://www.textmagic.com/

I have been using Brosix Personal for years. I am using it mostly to chat with my small circle of friends.  It does a great job ! http://www.brosix.com/

CD Messenger is a perfect office instant messenger for inter office communication. It’s a complete, secure and easy office instant messenger for company. Dozens features are available in a single application.

Features are : 

• Instant messaging for one to one, group and conferencing
• File Transfer
• Instant generation of printed transcripts of all your IM interactions
• Office notice board
• Configurable and customizable user interface
• Conversations history either department-wise or group-wise
• Group chat conferencing capability
• To-do lists (task management)
• Live spell checking in real time
• Create and send Sticky note
• Event alert management system
• Audible alarms to remind you about every important task
• SSL based encrypted communication
• Complete Administrative controls
• Access from anywhere

Elana... I totally forgot another really nice and knowledgeable person to help with website building: WebGizmos.

Sorry, Giz.