The S-series was originally a line of compact point-and-shoot cameras, slowly evolving into a prosumer line of cameras slotting right beneath the G series cameras. The S3 IS and S5 IS are still widely used as of 2012 because they are great cameras! The "S IS" series was discontinued and replaced by the "SX" series.
Canon Powershot S IS Series Cameras:
S1 IS, S2 IS, S3 IS, S5 IS