Besides Ed Trager's amazing font playground (a new version is in the work), I've enjoyed playing with Richard Ishida's Unicode pickers like the IPA one, creating word clouds with Jonathan Feinberg's wordle or admiring the colourful shapes of Ricard Marxer Piñón's Caligraft.
I find it very cool to see the functionality of these webapps being improved by the availability of open fonts like Gentium, Doulos SIL and many others in the background. Open fonts for an open web or something :-)
