I know you can play a whole crapload of games right in your browser using Java at
virtualnes.com. They aren't downloads, but at least you can pick up and play at anytime.
For games that require saving, or for other systems, you can get ROM emulators and ROMs pretty simply by doing a Google search. Try the Emulator Zone; they have an arcade section.
EDIT: Apparently,
MAME (Multiple Arcade Machine Emulator) is where it's at. The developer site even has some free ROM downloads.
EDIT2: romnation.net has a crapload of ROMs, including the ones you mention.
EDIT3: And, shit, some of those games you listed are awesome. I remember Qix! And Gyruss roxxorz!