Dr Mario - the game that will get your mum hooked - mine did
What you probably already know is that Dr. Mario is a tetris -like game that involves killing viruses by dropping pills on the viruses that match the colour of the virus. In order to actually kill the virus you have to make a line of four either vertically or horizontally. It's got cool music that for some reason you never get tired of.
The only weird thing to note is that although the game lets you go to level twenty from the menu screen, you can pass twenty, and indeed twenty one, twenty two and twenty three. But you will never pass twenty four because of the evil trolls. Actually no, when if you pass twenty four you pass to level...twenty four. Don't ask me why.
Also, passing levels that are a multiple of 5 (5, 10, 15, 20) in medium or high speeds will give you a little bonus animation with some twinkly music and some sort of animal/creature (from memory there's a turtle, witch, flying book, flying pig and UFO) flying over the three types of virus sitting on the tree.