At one of my favorite booths at GDC Play were two rather clever and entertaining iOS experiences: Circadia and Twirdie. The former is a sound and timing puzzler (as seen above), and the latter is a golf game that uses Twitter to determine the distance of a swing.

Simple Machine has certainly earned the #1 Music Game in the US position on iTunes with Circadia. Instead of riding the beat hitting wave of tons of tap games, Circadia essentially tasks players with timing the ripples of sound effects with a white dot elsewhere on the screen.

This is simple enough until the dot starts moving, and multiple ripples must overlap on the white dot. While the presentation has a charming minimalism about it, the mechanics gradually grow to a challenging frustration. There are currently 100 puzzles available for only 99 cents. A penny per brilliant stage isn't bad at all!

TUAW reports that developer Kurt Bieg is planning to add 25-50 new levels in an update, along with a zen mode, which would offer an infinite number of generated levels to play.

Twirdie was also available at his GDC Play booth. Bieg and Ramsey Nasser showcased the new 2 player pass and play mode, where players take turns "swinging." Swinging works by Twitter searching to see how many times a word was tweeted in the past 60 seconds, and the resulting number is how far the ball travels.

The resulting swings from typing foul and obscure words made for several laughs at the booth, making Twirdie feel like a great social game. Those who want to experiment with Twirdie can do so for free with Twirdie Lite.

Grab Circadia on iTunes and Twirdie on iTunes now and enjoy a double dose of interesting iTrickery.