If you're a fan of Wikipedia and own an iPhone or iPod touch check out Patrick Collison's Encyclopedia (US$7.99, iTunes Link). After launching the app you can download a complete copy of Wikipedia to your iPhone for offline viewing. It's great for people that fly a lot or otherwise would like to search the venerable online encyclopedia without a data connection.
To keep the size of the data file manageable it doesn't include images, references and user files.
Set aside about an hour to download the 2GB data file over a Wi-Fi connection and make sure to set the Auto-Lock Setting to never. If you interrupt the download (i.e. by hit the home button), it can safely be resumed.
If after resuming you don't see the download speed and the amount remaining under the status bar, hit home, launch the App again (confusingly called "Wikipedia" on the iPhone) and resume again. I had to interrupt and relaunch it a couple of times before eventually completing the download. Although that it may work over 3G, don't waste your time. Download the file over a Wi-Fi connection while connected to power.
It's fun to have a complete copy of Wikipedia on your iPhone and you can now settle a bar bet without having a data connection. It's also great for iPod touch users that are away from a Wi-Fi connection.
It's mildly disappointing that Dictionary costs US$8 (it really should cost US$5) and that it's not much faster than the online version of Wikipedia. The user interface could also use an overhaul but right now it's the only game in town.