Wikipanion is a Wikipedia client for iOS. You can search all of Wikipedia, also non English language versions. It works much quicker than using the mobile web version of Wikipedia.
Another plus: there are a lot of sharing options.
And the best thing: it’s free. There is also a paid version available (called Wikipanion Plus)
When I wrote this post, I was looking at version 1.6.5 for iPhone and iPod Touch. Requires iOS 3.0 or later. There is also an iPad version available.
Tried with iOS 4.0, 4,1 and 4.2 on an iPhone 3GS.