Street Projects, and possibly House Customization, when that happens. Street Projects aren't happening as of this minute though. You can also go to the Community Machine Rooms and use the equipment there (if they have fuel or you have some of your own) - there's a slight bonus.
Hold onto any blocks you have and bring them along to the Projects when they're going on, and donate them! It'll help expand a new area, and increase your percentage of the rewards!
Right now? Nothing. They're worth money, and the machines are often already fueled in the shops, but that's it for now. They were occasionally used in the street projects, but not since launch.
In the future they will likely be used for group hall construction and the house customization coming in 2012 sometime :)