DFRobot make one that is sold much cheaper than the official one, but it uses the official library because it has the same chipset (Wiznet 5100). Widely available on Ebay and many micro-controller vendors. The ones to avoid unless you're seriously strapped for cash are ones using the ENC28J60 chipset.
Typical example of the DFRobot board : http://cgi.ebay.co.uk/Arduino-Ethernet-Shield-(W5100-Chip)-_W0QQitemZ220592149599QQcmdZViewItem?rvr_id=&rvr_id=&cguid=d869b7ac11c0a0e204878047ffe56b99