Developer website: Dimitrij Mlekuz
Fast: Sitting in your dashboard, always ready to calibrate a radiocarbon date ... in a fraction of a second.
One sigma, two sigma or both: Reports one or two sigma ranges.
Pretty accurate algorithm: Does not just calculate intercepts but true probability distributions. For publications I still recommend a more standard programs like OxCal or Calib.
Calibration curve: Uses IntCal04 calibration curve (Reimer et al. 2004).
Free: Open source, released under GPL.