Did you search for a nice lightweight instant messaging application that can connect to multiple chat networks at once ? Here's my list of 3 best applications with one of them being a popular web client.

Adium

Released on September 2001 by Adam Iser- a then college student, Adium has grown up to be one of the most popular chat software available for the Mac OS x. Written using Cocoa API, Adium is registered under the GNU General Public License.Many protocols which the software supports through the libraries libezv (for Bonjour), MGTwitterEngine (for Twitter), and libpurple(for all other protocols) add on to its stability. One of the best things about the application is that it allows the user to customize anything and everything possible like buddylist window, chat window, sound sets, dock icon, status icons, plugins and apple scripts using resources its developers call "Xtras". Though initially Adium supported only AIM, it now supports multiple IM networks including MSN Messenger, Yahoo! Messenger, Google Talk, AIM, ICQ and Jabber.
Even when Adium is the biggest in its specific genre, one major short coming is that it does not support video chat or voice chat over a microphone.

Proteus

Aptly named after the mythological character,”Proteus” is versatile, capable of assuming many forms and flexible. The IM application written using the Cocoa API of Mac OS x and uses the IM framework libpurple attempts to bridge the gap between functionality and beauty-according to the developers.
Proteus supports multiple clients like Yahoo, MSN and Jabber, allows users to customize the IM window, fonts and sounds, allows user to add multiple ids for the same user and even supports file transfer. Proteus has the away messaging feature which forwards an important IM as SMS if the user is away. The full power of the Mac OS x could be applied while working with Proteus. The code is open and allows user to run shell applications while running the application easily as the software is based on UNIX.But like Adium even Proteus doesn’t support video chat.

Meebo + Mozilla Prism

Prism is a project of the Mozilla Fundation that recently entered beta. It lets users split web applications out of their browser and run them directly on their desktop.
Meebo is a popular web based chat client also using libpurple that allows you to have a central location from where to manage all your different network chats.

Resources

Download Adium
Download Proteus
Download Prism
Meebo Website