Yeah those have been around for years, don't know if they're any good or not.
One thing I DO know (as I noticed it on that same page) is that the Excaliber Space Invaders sucks. Big invaders, only one player, too easy and boring, don't bother with it unless you're just a hardcore collector (I'm a bit of a "bottom of the screen" shooter nut myself, but even *I* don't want that piece of junk!). My sister's kid has it, so yeah, I've played it. Bleah!