An Untitled Story
From author's description: "In An Untitled Story, you begin as an egg in your nest, and the rest is up to you to figure out. Fight 18 unique bosses, traverse a huge game world, and unravel a mysterious storyline. The single player game features non-linear progression and 5 difficulty levels for extra replay value. Also included is Heist Mode, a multiplayer capture-the-flag-style versus mode. Don't forget to hook up a USB gamepad before starting, to play it as it was meant to be played."
There's a excellent walkthrough by Targeted on the Helix Games forums available here (Local Mirror 24th Jan '08). Offer general tips, heart locations, ability locations, flower locations, and tips on some of the harder bosses.
Once you complete the game on the fourth difficulty level, the final difficulty level will be unlocked. At this difficulty, any hit will kill you instantly; hearts are only present for 100% completion.