This announcement is coming as a surprise since this third bundle hasn't yet crossed a million dollars (USD) in sales. In fact, the pace of sales has slowed down significantly and right now they're just at $724,442 in sales with just four days left. The last bundle had raked in $1.8M USD in sales and the first bundle also broke $1M. The third bundle may have a hard time generating a million bucks even though it's a nice collection of games, but Frozenbyte likely made the "bonus" announcement this morning in hopes of increasing the last minute sales volume.
Previous bundle games were open-sourced when breaking the million dollar milestone. What's being opened up now from Frozenbyte is Shadowgrounds and Shadowgrounds: Survivor, which is quite enticing. The engine source-code has been released but not the game content, so you still must purchase the titles to access the assets.
Also as part of today's bonuses is the playable sneak preview of their Splot game and a level editor for the Trine game, also included as part of the bundle. The announcement can be found on the Frozenbyte blog.
The Humble Indie "Frozenbyte" Bundle with the Shadowgrounds, Shadowgrounds: Survivor, Splot, Trine, and Jack Claw games can be purchased through next Tuesday at HumbleBundle.com.