There are a number of client implementations, but only the Python GTK2 and the HTML5 clients are really usable at present. However, fixing or adding new clients is relatively easy as most of the features are optional and the client code is much simpler than the server side. Alternatively, you can always write a new client.
Python GTK2 Client
This is the default client, fully supported, the only full featured client.
GLPixmapBacking for drawing window contents. (see wiki/ClientRendering for details)
OpenGL should be used by default when supported.
Settings are accessible through a system tray menu.
Python GTK3 Client
Work in progress ticket #640: this client is not yet usable, it does not support keyboard input yet and lacks a number of features, it is also slower than the GTK2 client.
The clients below are not officially supported:
Python Qt4 Client
Defunct ticket #388: this client has been removed from the codebase because of a lack of maintenance.