For $10, Put Wikipedia In Your Pocket
WikiPock takes up about 4GB of storage, and is currently available for Windows Mobile and Blackberry devices. It's $9.99 for the basic version, but you can opt for a version that includes updates for $14.99. You can also get a pre-loaded 8 GB microSD card if you'd prefer for $15 extra (4GB microSD for $10 extra). An iPhone version is in the works, but at 4GB of required storage, that's quite a hunk of the iPhone memory (since it still strangely, ahem, does have any external storage slots). Note that WikiPock doesn't include images.
Ten percent of all WikiPock sales will be donated to the Wikimedia Foundation.
Why would you want these apps when you can just surf over to the site? Speed of searching, really. Obviously, it's a lot quicker to find information if you don't have to hit the site.
We actually like the idea of WikiPock's updateable version, but it's not yet available for the iPhone, and 4GB of RAM is quite a bit to take out of an iPhone's storage. We'll reserve judgment until they release it; perhaps they will cut the storage requirements down for the iPhone.