The PGA Tour's Las Vegas Senior Classic is held each April in nearby Summerlin, and the Las Vegas Invitational takes place in Las Vegas each October. The National Finals Rodeo is held in UNLV's Thomas and Mack Center each December. From time to time, you'll find NBA exhibition games, professional ice-skating tournaments, or gymnastics exhibitions. Finally, Las Vegas is well-known as a major location for boxing matches. These are held in several Strip hotels, most often at Caesars or the MGM Grand, but sometimes at the Mirage.
Tickets are hard to come by and quite expensive.