Emulating games on an Android device is something you might have seen in YouTube videos, with older titles especially. Some of them make their way to the mobile platform officially, and Galaga is one of those examples.
While the game might not be instantly recognizable in the same way as Space Invaders or Super Mario Bros. might be, it does have something of a cult following, and it has had a few re-releases of its own.
It apparently supports Android 2.0 and up, and that's a lot of Android devices! Even my phone's 320x480 display is considered compatible.
Oh, and it's free. Completely, 100% free. Shame about the advertisements - according to Google Play reviews, adverts can be obtrusive mid-game. That, and it's hard to tell what devices work well and what don't. One comment complains it isn't smooth on a Galaxy Nexus
(would be great on Android 2.0 hardware, then).
Galaxy SII and SIII owners have not complained about performance in the reviews, but that isn't really a full picture.
Anyone who might not remember Galaga, but did play Ridge Racer Revolution should recall the following bonus:
Source: Google Play