If you are a new ubuntu user who just came from windows or Mac os to the Linux and open source world then you might be searching for some of the equivalent a open source applications in contrast to your favorite one(That you enjoyed on Windows or MacOSX).In fact a bunch of Instant Messaging clients are available for Ubuntu 10.04 LTS.
Empathy IM Client : The Default Instant Messenger for Ubuntu 10.04 Lucid Lynx
Empathy Instant Messenger is already installed(From the beginning itself if you have installed the Ubuntu 10.04 Lucid Lynx from a standard Live CD).So to send and receive message go to Applications->Internet->Empathy IM client and enter the account details(gmail,yahoo,facebook….) to begin chat.It does not work only as an instant messenger but you can also enjoy Internet calls(If your friends on the other end have also installed Empathy IM client) and file sharing.
Other two great Applications that you may like to install on Ubuntu 10.04 are –
Gtalk : Gtalk for Ubuntu 10.04
Skype : Install Skype in Ubuntu 10.04