Just one look at the screenshot tells you what you're dealing with in Invaders World Tour: invaders, of the space kind. The game is a really well-done Space Invaders clone, with colorful graphics, an interesting tilt-to-move mechanic, and power-ups to help you take out the invading menace. The game just got overhauled, too; it now boasts OpenFeint integration, new graphics and mini games, new enemies, and lots more. At just 99 cents, it's a steal for retro arcade fans.
If you want to try a free title, you can give Invaders World Tour USA a shot; it won't have all of the shiny new features of the main game, but it's essentially the main gameplay in a free trial version. Either way, there's fun to be had here, so go grab the free app, or pay the buck for more.