Recently there has been several implementations of 3D and 2.5D (isometric) rendering of OSM. Honestly, I haven't been enthusiastic by any of them. One of the reason is that apparently no one is trying to actually represent streets (the S of OSM) better than flat spaghettis.
I've just seen this map, and I got faith again that 2.5D representation can be useful and beautiful: http://gz.o.cn/
This is the map of Guangzhou (China). Several other big Chinese cities are also available. There are so many amazing things in this map: * multiple level streets are (almost) readable. * building shapes and texture are so precise that each remarkable building is distinguishable on the map. * POI are active: they are not just a big stamp on top of the map, they can be shown or hidden, and react to mouse hover. * It's so amazing complete (fair enough, the quality of London in OSM is probably almost on par) * It makes any city look like a city from SimCity 2000 ;-)
Hopefully eventually it will be possible to see OSM data rendered as such 2.5D quality map!