Many come to Vegas for the gambling and nightlife. I come for the history.
Mob Museum was a must-see on my Vegas visit last summer. At work, my friend and I implemented Mafia Monday, Tough Guy Tuesday, and Wiseguy Wednesday so we're pretty much designated mob men by now.
Get ready to spend a good few hours here as the museum is three (or 4, I forget) stories packed to the brim with mob history and ties it into the history of Las Vegas. A lot of interactive exhibits make it a fun museum for all. Pick up a fedora in the gift shop while you're at it. Seriously, they're loaded.
One of the most fascinating pieces of history I think has to be the St. Valentine's Day Massacre wall. That was a trip to see.
If you're in town then definitely check it out. You won't be disappointed.