Are there different Ethernet shields available?
Yes, although none of them are quite as elegant as the official Ethernet Shield solution and may not work with all Arduino boards.

One option is the WIZ820io ethernet module at about $20. There's a great discussion on the adafruit forums regarding its use.

A more involved DIY choice is a shield based on the cheap (just $3) ENC28J60. A blog post at Open Electronics details the building and use of the shield while you can buy what appears to be a fully assembled version for ~$26.

There's a well-reviewed shield implementation of the ENC28J60 from DealExtreme that goes for about $18.

DealExtreme has another option (again based on the ENC28J60) that's smaller and not a shield. The board comes fully assembled for $10. It connects directly to the Arduino's SPI pins.

Source: 4 Arduino Ethernet Shield Alternatives