Something that leaps to mind seeing most of these is that they have really tried to create game, but have limited skills and resources to do so. As a result they tend to look poor quality, play with difficulty, or be instantly dated. They simply lack the polish and design flair most people would expect from a game. Some of the best are very simple, such as the Liberation referencing tutorial - where a cartoonist has drawn a number of cats which have been tied together by a detailed, but straightforward, tutorial. They haven't tried to create something that looks like a commercial game, but instead used what game elements they could successfully and in a way that looks good.
A very simple online board game from The University of Northern Carolina, The Information Literacy game is available under a creative commons licence.
"Players take turns moving around the board, answering questions. There are four categories, and two questions must be answered correctly from each category in order to win."
It can be slow to load, but it should work if you give it time...
A simple game created using Quia, where players have to answer questions related to definitions of different types of sources to "win a million dollars". Ready reference shows a simple but compelling type of game play where you are tempted to try and beat your previous best score and get to the million dollar prize!
A bit unfair, me calling this "the cat cruelty game", but it is tempting for some players...
You learn about referencing with Harvey the cat in this Liberation: referencing tutorial. The game like element comes with the reaction to your cartoon cat when you answer correctly or incorrectly. For each correct answer, Harvey gets a treat... but if you get too many wrong he starts to look a bit bedraggled and neglected.
Created using Adobe Captivate (so a flash file), Librarian or android is essentially a quiz that sends you outside the screen to search for information. Only by answering the questions correctly can you progress through the quiz.