It's probably 2 projectors at an angle and a large plastic/glass surface. Possibly even 2 layers - glass or plastic on top of something more reflective for the 3D effect.
EDIT: Nope. It's a mylar screen tilted at 45 degrees. Here's a demo video from AV Concepts, one of the companies that did the Tupac thing (the other company was the one that did the Gorillaz thing I posted above):
For the projection aspect, a San Diego company called AV Concepts used a variation of a visual effect that was discovered in the 19th century, known as Pepper's Ghost.
Though the projected image has been widely described as a "hologram," it is a 2-D image and not a hologram, which is 3-D.
The effect was first used in an 1862 dramatization of Charles Dickens' novella "The Haunted Man and the Ghost's Bargain," staged at the Royal Polytechnic Institute in London, according to Jim Steinmeyer, an illusion designer who has written extensively about the history of his craft, including Pepper's Ghost.
The effect relies on an angled piece of glass in which a "ghostly" image is reflected. "A piece of glass can be both transparent and reflective at the same time, depending on how it's situated relative to the audience," said Mr. Steinmeyer, pointing out the secret.
In the Victorian version of the trick, the glass reflected an actual actor, situated out of sight in near the orchestra. On Sunday night, the image was projected on a piece of Mylar—a highly reflective, lightweight plastic—stretched on a clear frame.
This one reminds me of the holographic librarian in The Time Machine. I think Arsenio Hall played him, and he would go from projection panel to projection panel as he followed Guy Pearce's character around the library.