Mine is Alley Cat - a computer game from 1984 (*points at her 'alleycat' user icon* ;), created by Bill Williams. As a mischievous alley cat, you have to sneak into apartments and create havoc, evade obstacles, and compete for the love of your sweetheart. It's amazing how much functionality is packed into such a tiny (by the modern standards) file! The graphics still looks pretty stylish, even that it's 4 colors CGA (remember how one had to be so inventive with the design, switching between 2 palettes - white, cyan and magenta vs red, yellow and green) And the music is so catchy. My favourite screen is the screen with the dancing kittens - the bonus for completing the 2nd and the 3rd levels (I had never succeeded to progress further).
At the beginning of my acquaintance with the IBM PC it used to be my favourite game... now not anymore, but sometimes I just can't resist going back in time (thanks to the DOS emulators) and engaging in some simple, cheerful and relaxing alleycatness!
(AFAIK the game is now freeware and can be downloaded from zillions of places)