My friends have gotten good deals to shows through Tixs4Tonight.
You can get tickets to shows the day before and on the day of for up to 50% off. However, Cirque and Le Reve shows seem to be only up to 20% off and you can only buy their tickets on the day of. They don't sell "O" tixs. They also sell 20% off dining vouchers, but we didn't care for any of the restaurants on their list.
Tixs4Tonight was a hassle as we ordered the tixs on the phone, then had to go to their ticket office (thankfully no lines at the time) to pick up vouchers, then go to Wynn 1 hour before the show to exchange them for Le Reve show tickets.
Try Vegas.com, Groupon, or even Travelzoo beforehand for shows that you want to see.