Zotac’s GTX Titan Amp! Edition is an overclocked variant of the reference GTX Titan that includes the company’s "Boost Premium software" and Assassin's Creed bundles.
The Zotac GeForce GTX Titan Amp! Edition features a very modest overclock of the graphics card’s core and memory clock, but is otherwise identical to Nvidia’s reference GTX Titan.
Reference GTX Titan
GTX Titan Amp! Edition
Core Clock (Base)
Core Clock (Boost)
It is also worth noting that the GTX Titan Amp! Edition comes with two free bundles. The “Zotac Boost Premium software” bundle includes Nero Kwikmedia, Unigine Heaven DX11 Benchmark, and XBMC. The Zotac Assassin's Creed 3-Game pack provides a free copy of Assassin's Creed, Assassin's Creed II and Assassin's Creed: Revelations.