(updated Oct 28, 2023)
When you check in to a hotel, they usually ask you for a credit card to have on file for incidentals and as a security deposit.  When they do that, they pre-authorize the card for additional charges. The amount varies from hotel to hotel and is usually multiplied by the number of nights you are staying.
This has been standard practice for a long time, but I have heard multiple stories recently of people being caught off-guard by the pre-authorizations as well as not fully understanding what they are.  The pre-authorizations or “holds” are not charges. They simply reserve a certain dollar amount on your card so that it is available when you check out and pay your bill. It is similar to what gas stations do when you pay at the pump. The pre-authorization is not an actual charge, but it does reduce the amount of credit available on your card.  This reduced available credit balance is very important if you are using a debit/ATM card.  If you use a debit card for the room hold, it effectively lowers your bank account balance. This can lead to bounced checks and big headaches. For this reason, I highly discourage using a debit card for the room hold. The hold is usually released within a few days of checking out, but it can take up to 10 days for your available credit or bank balance to get back to normal.

I always use an American Express card with no preset spending limit for the room holds specifically to avoid any possible credit limit hassles. You can always use cash or a different card to pay the room bill when you check out.

If you do not have a card to put on file for room charges, some hotels will require a cash deposit. Others will still allow you to check in but won’t allow you to make room charges. For example, Aria requires that you pay the full room balance as well as put down a $200 security deposit if you do not want to give them a credit or debit card. In that particular case, they won’t allow you to charge anything to the room either. The policies vary from hotel to hotel. Call your hotel before you arrive to see what the options are if you will not be able to give them a credit card when you check in.

• Aria  $150 (or $200 cash deposit per stay, but you can’t do room charges with the cash deposit)
• The Artisan $150 deposit per stay
• Bellagio  $150 (with $200 minimum hold)
• Caesars Palace $150 (or $200 cash deposit per stay)
• California Hotel $100 per stay
• Circus Circus $35
• The Cosmopolitan $150
• The Cromwell $150
• The D $100
• Encore $150 credit card hold or $300 cash deposit per night (yes; per night)
• Excalibur $50
• The Flamingo  $150 (or $200 cash deposit per stay)
• Fremont pre-authorized for full room and tax plus $100
• Gold Coast $100 per stay
• Golden Nugget $50 per stay (unspecified, but higher amount held for suites)
• Harrah’s $150 (or $200 cash deposit per stay)
• Horseshoe (formerly Bally’s) $150 (or $200 cash deposit per stay)
  
• The LINQ  $150
• Luxor $50
• Main Street Station  $100 per stay
• Mandalay Bay  $100
• MGM Grand $100
• Mirage $100
• New York – New York $50
• NoMad $100
• The Orleans $100 per stay
• Palazzo $150
• The Palms $50 for regular room, $100 for a suite
• Paris  $150 (or $200 cash deposit per stay)
• ParkMGM $100
• Planet Hollywood $150 (or $200 cash deposit per stay)
• Red Rock Resort $100
• Rio $150 (or $200 cash deposit per stay)
• Signature at MGM Grand $150 ($300 max per stay;  Prime Suites have a $300/nt hold during the week and $500/nt  during the weekend)
• Sahara $50
• The Strat $50  ($200 max)
• TI (Treasure Island) $100
• Vdara $100
• Venetian $150
• Westgate  (formerly the Las Vegas Hilton, LVH) $50
• Wynn $150 credit card hold or $300 cash deposit per night (yes; per night)