Connect Four

Circle Defense

Pong

This one has a slight issue with scaling, so it's a little large.  However, it should play just fine, and you have 5 guesses as to what the theme is...

Tetris

Tetris is a classic, so I will assume you know the gist of how to play it!  The music doesn't loop, so it will get a little quiet after some time.

Ultimate Space Shooter of Doom

Because it's always better if it's of doom! The game starts from the get go, so get ready!

Due to my foolish ways in my youth, this game is heavily dependent on its resolution size to function properly. Thus, it's a bit big (you'll probably want to scroll horizontally a bit to see the whole screen).