The 108 bus
The RTC bus that goes to and from McCarren Airport from the strip is the 108 bus. You can get on the 108 bus at the airport in Terminal One's "Level Zero" (there are signs at the airport directing you to the Zero Level / ground transportation). The 108 bus will get you all the way up the Las Vegas strip (running a couple blocks east of the strip on Paradise Avenue) and will take you all the way to the Fremont District, dropping you off near the Four Queens on Fremont Street. The 108 bus runs daily from 5 AM until 1:30 AM, and runs every 30 minutes. The fare for the 108 bus is only two dollars, but your better bet is to get a 24-hour pass for $8 or a 3-day pass for $20.

The 109 bus
If you are staying on the south side of the Las Vegas strip, another option for getting to and from McCarren Airport is the 109 bus, which also can be caught in Terminal One's "Level Zero" bus stop. The 109 bus will take you to the South Strip Transfer Terminal where you can transfer to the SDX bus which will take you directly to the South end of the Las Vegas Strip, at which point it cruises north on the strip to the north end.

You can purchase a 24-hour pass or a 3-day pass at most RTC bus stops on the ticket kiosks. The 24-hour and 3-day passes are good for any regional RTC bus in Las Vegas plus the Deuce bus which runs up and down the Las Vegas strip all the way to the Fremont Experience, 24 hours a day, every 15 minutes. (Read more about the Deuce bus here.) Another notable RTC bus that cruises up and down the strip is the SDX bus, which runs daily from 9AM until midnight.

If you are traveling with a lot of luggage you may be better off using a shuttle to get to and from the Las Vegas airport, but if you have only one or two bags you really can't beat the 108 bus. Shuttles to and from the airport generally cost $8 per rider for a one way trip, and the shuttles are actually slower than the 108 bus.

All in all, if you plan on visiting the Las Vegas Strip and are looking for inexpensive transportation, don't waste your money on taxis or shuttles, take the bus instead! It's well worth it. More information on the RTC bus system can be found at the official RTC website.