Cheapest I've seen it online and offline (Most retailers £24.99+).
Product linked seems to be the DS Lite version.
Lets you browse the web with your Nintendo DS and WiFi thanks to its Opera powered browser.
Includes DS Browser cart and Nintendo DS Memory Expansion cart (that goes into the GBA slot)
The DS version works on both DS old and DS Lite models (but the expansion cart protrudes out of the GBA slot). The DS Lite version works only on the DS Lite (the expansion cart is a smaller to fit in the Lite properly)
More importantly, the Nintendo DS Browser works over the Nintendo WiFi Connection ('The Cloud' and 'BT' wi-fi hotspots across the UK), so access to the internet via these hotspots on the DS is FREE!