Very few hotels offer free Wi-Fi access. In general, either there's a specific charge for Internet access during your stay (see our listings for Internet charges), or else it's included as part of the so-called "resort fee".
Those hotels that do offer free Wi-Fi tend to include those non-casino, long-stay properties aimed more at business travelers, like La Quinta, Fairfield Inn, Best Western, etc., while exceptions include the Gold Spike downtown and Wild Wild West. Otherwise, a number of hotels have one or more "hotspots" where you can get online -- the Terrace Pointe Cafe and Pizza Place at Wynn, for example, or numerous locations throughout Tuscany. A number of the Cool Pools also have free Wi-Fi access, but they're only open seasonally.
Since we last updated our listings of Free Wi-Fi hotspots, there have been a number of casualties, including Aces & Eights bar, Cafe Mitz, and reJAVAnate. Click the link for the full listings, and let us know if you know of anywhere we missed.