My little one is 4 and loves playing on the PC, i have a couple of games she likes but mostly they are a bit too complicated and only interesting because of the movement and sounds her hitting the keyboard results in :-)I recently found Childsplay which is a tool you simply have to download and install for your todler. The interface is simple and the games are entertaining. Above this they are extremely educational and addictive.
I couldnt believe how quickly my little one improved her mouse usage ability, once forced to improve in order to get 'the game to work', as she says.
There are sounds and pictures and memory games.... And it is free as it is opensource.
Head over to their site and download a copy for either Windows or Linux.
Some screenshots; 
