Anyone have a link for a good guide on how to build a Flex app to work with Open source REST services? I am familiar with how to do it in ESRI and Arc Server Manager, but don't want to pay for a ArcServer license.
You might wanna take look at http://openscales.org/ never used myself i always used JS or ESRI silverlight, but should be quite similar to the openlayers API. It support OSM, WMS, wmts , WFS-T... So with a Geoserver (or any other OGC compliant GISserver) backend you should be able to build similar apps as with esri stuff.