One of the best reasons to come to a conference like GDC Online (which I'm at this week in Austin, Texas) is touching base with others in the iPhone and iPad community and sharing your favorite apps with each other. I've been hooking as many people as I can on Pocket Frogs (I just passed 50% of awards completed, and the app just added Game Center integration in a new update), and my colleague Jeff Scott of 148Apps recommended Blue Defense: Second Wave! to me. Blue Defense is a top-down shooter with an iPhone twist -- instead of moving your ship around as it shoots down enemy ships in 2D, you tilt your iPhone (or just touch the screen) to aim a planetary cannon, taking out ships as they come to you.
The tilt interface is very intuitive, and there are even some fun multitouch functions. You can split your fire by double-tapping anywhere to create fixed aiming sights, or even grab and drag away from the planet to shoot a gigantic screen-clearing gun. This is actually the second version of the game, so there are multiple modes to get through, including two infinite modes that you can use to try and raise your score up on the game's leaderboards. There are also lots of levels to play and medals to earn while doing so. The graphics are 2D, with a simple but smooth and well-designed look, and the Retina Display makes the game look great.
Blue Defense: Second Wave! is an excellent title for just US$1.99. It's also a universal build, so I bet it'll look even better on the iPad. I've heard some other good app recommendations from the panels and attendees here at the show, so stay tuned all week for more.