I recall having seen this before, but it was around a year ago. Have you found Mapbox's editor already? You can even import kml's, csv's and geojson's and save the map output for embedding.
Edit: looking around more it seems like Mapbox is a sort-of alternative to GIS. It makes handling Openstreetmaps data and adding some of your own data relatively easy. Interesting.
Edit 2: Okay this is pretty cool and definitely a skill I want to develop / learn. Works pretty good with ArcGIS as well. Will look into this more. Thanks for pointing it out, kb and insomniasexx :)
P.S. any interest in a Hubski mapbox skin?