There's kiwix:

• Works on every computer platform. First you have to install a little program, then download the current dump of wikipedia in your language. (no more than 10 GB).

There's Offline Wiki:

• A application for the chrome browser. You get to choose between 2 sizes, a ridiculously small 14 MB one, and a larger but still surprisingly small 1 GB one. However it's text only, no pictures.

There's WikiDroyd:

• For android. It's free and has tons of advanced functions like history, favorites, random articles, sorting, etc. There's even an optional text to speech but that requires an extra download.

There's Wiki Offline:

• For iOS devices. Very user friendly and neat looking but costs a whopping 10 bucks.

There's WikiPock:

• For Symbian, Blackberry and windows mobile, also costs 10$.

The official android and iOS wiki apps both support saving of pages for offline reading, but it's unlikely that you'll know beforehand which particular article you'll need.

Here's the official wiki about their offline possibilities.

Although you're probably looking for dumps.wikimedia.org.