For years, 25,000 miles has been the standard for US domestic award tickets. However, some new "sweet spots" in airline award charts are slowly moving that number down. Here are some frequent flyer programs to consider:
Domestic American Airlines round trips start at just 9,000 miles for non-stop flights that are less than 650 miles each way. For 15,000 miles, you can fly up to 1,151 miles each way. For 20,000 miles, you can fly up to 2,000 miles each way. Flights with stops require combining the above amounts. For example, if one connection is 500 miles and the other is 1,000 miles, you could redeem 24,000 miles for a round trip.
You can transfer to BA from both Ultimate Rewards and Membership Rewards. BA also doesn't charge close-in award fees or fuel surcharges for AA flights, so they're great for last minute trips!
Frontier is one of the final domestic hold-outs still offering domestic round trips for 20,000 miles, no strings attached. You need to fly through Denver, though, and their destinations are limited.
ANA offers domestic flights on United (and, for a while, US Airways) for 20,000 ANA miles as long as the combined mileage for your round-trip flights is under 2,000 miles. You can fly up to 4,000 miles for 22,000 ANA miles.
ANA does not charge fuel surcharges for flights on United, and you can transfer points to ANA from Membership Rewards.
Shorter-distance domestic itineraries (including between the US and Canada) are often priced at 15k, but you need to fly only Air Canada to get the discount. They often charge outrageous fuel surcharges.
Short flights (under 700 miles each way) are priced at 20,000 miles for a round trip. No strings attached.
If you're a Citi AAdvantage card holder, don't overlook AA's Reduced Mileage Awards, which offer round trips to select destinations for only 17,500 miles! The only downside is you need to call to book, and you'll invariably incur a telephone booking fee.
If you have their credit card and book a trip using only US Airways operated flights, you'll get domestic round trips for 20,000 miles.
Spirit, AirTran, Southwest, and other low cost carriers typically offer redemptions based on the price of the ticket. There are definitely times when you can fly for under 25k miles (or equivalent).
It's probably worth noting that Starwood transfers to airlines come with a 5k bonus for every 20k points transferred, so you can effectively get round trips on any airline for 20k or under.
Summary of Round Trip Prices by Alliance
Star Alliance: 15k on Aeroplan, 20k on ANA, 20k on United, 20k on US Airways (with credit card).
Oneworld: 9k on BA, 17.5k on AA (with credit card).
None: 20k on Frontier, various amounts on LCCs.