I got hooked on Sporacle lately. They've got hundreds of free trivia games which are a great way to pass some free time while developing various skills in memory, science, geography and history.
This is one of the tougher geography games I've tried. It took me a week to get all 53 right. I've done the Europe and other quizzes but this one is the toughest because so many look alike. (Although Europe has some tricky flags too, like Andorra / Moldova and Poland / Monaco.)
http://www.sporcle.com/games/africaflags.php