MAPS finder chart engine
Brian Casey's Astronomy Tools:
Hourly Airmass Tables
Daily Almanac
Object Seasonal Observability
Sky Chart for CTIO
Recent Transients
Gamma-Ray Bursts: http://gcn.gsfc.nasa.gov/gcn3_archive.html
Supernovae: http://www.cfa.harvard.edu/iau/lists/RecentSupernovae.html