You can check in online 24 hours before your flight leaves. This allows you to print out your boarding passes at home and go straight to bag check at the airport, or straight to security if you're not checking baggage. Some airlines may provide special incentives for checking in online - for example, Delta waives their curbside bag check fee, and Airtran and Spirit charge lower baggage fees online.
To check in, you’ll need either an e-ticket number or a confirmation code - both of which should be in an email or set of papers you got from the airline or the group you’re traveling with. You may also need to know your flight number and/or the airports you’re starting and ending at.
If you can’t check in online, or you don’t want to, you may be able to use the same information at an automated check-in kiosk. This may vary depending on the airline and the airport. You can always wait in line and check in with a human being too.
Here's some checkin links for you:
Airtran Airways
Alaska Airlines/Horizon Air
American Airlines
Continental Airlines
Delta Air Lines
Frontier Airlines
JetBlue Airlines
Midwest Airlines
Northwest Airlines
Southwest Airlines
Spirit Air
United Airlines
US Airways
