People tend to stick to the more known messaging clients when there are so many excellent open source alternatives out there like Psi.
This client was built to connect to multiple Jabber accounts and more known IM accounts like Yahoo or MSN. It's simple to set up, thanks to startup tips and guides found in the forum and wiki.
This last point is one of the main strengths of Psi. The open source project benefits from a strong community which has really helped make it a solid messaging client over the years. One of the examples of that is all the localized version of Psi that are available.
Psi is easy to use, yet discreet and features lots of iconsets and emoticons. You can also fully customize the interface and add your own icons. You can work with hotkeys and create command lines to insert in your chats, however more technical users will probably feel more comfortable working with these.
Users worried about their security or working with Psi in a public environment will be glad to know that it features SSL encryption and lets you use OpenPGP to send encrypted messages.
Psi is a real pleasure to use and is loaded with features, from security to customization and not forgetting multiple IM and Jabber compatibility. If you're looking for a good alternative messaging client, this is the one to choose.