One of my favorite PC games on my cab is "Warblade". It's Galag with a twist:
As you shoot things some "power ups" drop from some of the ships you destroy. Power ups include (but not limited to) money, double shott, triple shot, speed, bombs (destroys everything left on the screen), 2x and 5x point multipliers... etc...
There are really lots of variations for example of of the bomb is the "gem bomb" which turns every reamining enemy on your screen into a small gem that starts falling off the screen and that you can collectr for more money.
in between stages you can enter the store and buy additional power ups with the miney you collected.
Some power ups also start "mini games" like the Meteor Shower where you try and navigate a meteor field for as long as you can to gain bonus points.
The levels and enemies are varied , the music is quite good (you can download the MP3s on the site)
Maybe some Galaga "purists" will be horrified at having it "evolved" that way but I love it, it still has the same feel as the original, jsut better IMHO.
You can download a free demo or get the full game for $15, including free upgrades (the game is till in active development).
The music is pretty good too (you can download it from their site as mp3s)
As far as controls go, the regular MAME controls will work, however you may need a mouse/trackball to navigate the menus (unfortunately my cab is down for maintenance atm so I can't check).
Check is out here:
http://www.warblade.as/I belive you can even create your own levels but I have never played with that.