Audio commentaryThere's a unlockable commentary track by Toby Gard and Jason Botta in which they not only talk about the making of the remake but also about the original Tomb Raider. It's also possible to unlock the Lara model from the original Tomb Raider and play with her.
Core Design versionA different version of the game was under development by Core Design, the developers of the older Tomb Raider games, but Eidos called Crystal Dynamics to do it simultaneously. Core Design's version was ultimately scrapped completely.
ReferencesAs it already happened in Legend, once again the folks at Crystal Dynamics take a chance to tip their hat to their own Legacy of Kain series: if you find level 12's relic (The Great Pyramid) you'll unlock the wetsuit Lara wore back in Tomb Raider II, only this time it has a b&w logo on the chest. You might recognize this logo as Raziel's clan symbol.
Information also contributed by Dr. M. "Schadenfreude" Von Katze