There's an extended discussion of the pros and cons of the already mentioned http://www.sparkfun.com/products/9473
at that page.
The biggest "con" seems to be the two double rows of header pins... makes connecting via breadboard at least difficult, and probably impossible.
The price: $25.
A "proper" shield: $46
$21 is $21... but you have to count your time as worth SOMETHING!!!
I like "simple" too... but sometimes "simple" is buying the better board in the first place.
The Ethernet shield Sparkfun is selling... when they can get it in stock... also has a microSD slot, which opens numerous possibilities...
See also (not tested by me, but looks fine... $32...http://www.seeedstudio.com/depot/wiznet-ethernet-shield-w5100-p-518.html
A shield. No memory card slot.
Take care when buying Ethernet shields that you have what is required by way of power. If you are not using a "standard" Arduino, you may not have a ready source of 3v3, and some Ethernet shields need it.
Take care if buying an old shield that reset issues are not going to annoy you.
If the board is not W5100 based, be sure you are prepared to work with a less extensively supported chip.
$37, W5100, some kind of memory card slot...http://www.bizoner.com/arduino-duemilanove-wiznet-w5100-ethernet-shield-p-209.html
$45, with POE for Ethernet and attached Arduino... lots of encouraging technical notes..http://www.freetronics.com/products/ethernet-shield-with-poe
$32, with microSD, W5100... looks like it might be a version from the "official" Arduino stable...http://www.freetronics.com/products/ethernet-shield-with-poe
.. seems to be the same board, $40, more information.
... AND... at...http://www.aliexpress.com/fm-store/604119/210881751-465060173/Arduino-Ethernet-Shield-Wiznet-W5100.html
... there's a suggestion that this board comes from tinyos electronics, China.
So many options! So little time!