Post about Streeteasy switching from Google Maps

https://plus.google.com/118383351194421484817/posts/foj5A1fURGt

One of the biggest tools mentioned is TileMill, put out by MapBox

http://mapbox.com/tilemill/

And Open Street Maps

http://www.openstreetmap.org/

It looks like Foursqaure very recently made a similar move, also using TileMill/MapBox tools

http://blog.foursquare.com/2012/02/29/foursquare-is-joining-the-openstreetmap-movement-say-hi-to-pretty-new-maps/