Minecraft wiki entry for Development_versions, associated text: Improved the system.
"Useful to map makers, modders, players and server admins".
Added a way to do things when clicking things in chat, like executing a command.
Other actions include opening a URL and showing an item tooltip when hovered on.
"Saved screenshot as" messages can now be clicked to open the screenshot, takes players to the .minecraft folder.
Names can now be clicked to PM people.
Added /tellraw <player> <raw json message> to send chat messages with custom functionality to players - sender will not appear.
Syntax example: /tellraw @p {text:"Daytime? ",color:"green",extra:[{text:"Click here!",color:"blue",clickEvent:{action:"run_command",value:"/time set day"}},{text:" Nighttime? "},{text:"Click here!",color:"blue",clickEvent:{action:"run_command",value:"/time set night"}},{text:" Have fun!"}]}.