When I first moved from Windows Mobile to Android, I was looking to replace all of my apps with something similar in Android.
One such app was Tomeraider which allowed me to download a file containing the entire Wikipedia (minus pictures) and look-up anything that crosses my mind, anytime, anywhere, without the need for an internet connection.
Tomeraider does have an app for Android, and it's a pay app, but unfortunately, it doesn't work.
Luckily, there's another, much better app that surpasses Tomeraider (and actually works!). It's WikiDroyd. You can download the entire Wikipedia in any of 28 languages. The English database takes up about 6gb, and was last updated on March 12, 2010. Still no support for pictures, but perhaps that's coming?