From the Indie Royale site:
Headlining the bundle in an acclaimed tower-defense game with a free built-in expansion (for Windows on Steam, Desura, and DRM-free). Featured alongside it is a retro turn-based dungeon crawler (for Windows and Mac in a Steam debut, Windows and Linux on Desura, and Windows/Mac/Linux DRM-free), a pair of titles from the same developer spanning platformer shooter and horizontal shmup (for Windows on Steam, Desura, and DRM-free), and an incendiary 2D action platformer (for Windows on Desura and DRM-free).
Those who pre-order the April Fools Bundle receive an awesome vocoder-heavy bonus: Sexy Synthesizer's Japanese '80s-style chip album Rock: Deluxe Edition, available in .FLAC and HQ .MP3 file formats. Those who pay over the minimum while the bundle is live will also receive the bonus."
Current speculation puts the games as:
- Defense Grid: Awakening
- Hack, Slash, Loot
- Alien Zombie Megadeath
- Astro Tripper
But as they haven't been confirmed, take them with a grain of salt (the only game in that list currently with a Desura page is Hack, Slash, Loot).