Halcyon is an ActionScript 3 rendering engine for OSM data. Halcyon is used as the map display engine for Potlatch 2, as well as being used in other projects.
Halcyon can be used to read from many different sources of OSM data - XML connections to live OSM servers, read-only .osm files and even AMF connections. Halcyon also supports Vector Background layers, which Potlatch 2 uses for shapefile and GPX display, as well as tiled image backgrounds.
Halcyon uses MapCSS for specifying stylesheets, allowing you to control the display of the data in your application without needing to recompile halcyon. A demo of the stylesheet system is available at geowiki.com
Most of the halcyon development is done as part of the Potlatch2 project, but it's worth noting that halcyon is pure AS3 and doesn't depend on Flex. You can get the sourcecode for halcyon as part of the potlatch2 project from github. See Potlatch 2/Developer Documentation.
See the README.txt file for instructions on compiling the standalone halcyon viewer.