Nifty Node.js+MongoDB demo, originally written by @seb_m.
To deploy it on DotCloud:
git clone git://github.com/jpetazzo/MongoPop.git dotcloud push mongopop MongoPop
Note that you have to wait a few minutes for the MongoDB instance to be ready (it preallocates a few gigabytes of space, which takes some time). Forthcoming version of the MongoDB service on DotCloud will deploy faster, but meanwhile, give it some time :-)