Yes indeed, most US airlines charge for all check baggage.  There are exceptions if you have "status" with either the airline themselves or with any of their affiliated airlines (eg, Star Allaince, OneWorld, etc), or if you're flying in First Class, but for most travelers you'll be charged to check any bags.

The fees vary from airline to airline, but are generally fairly similar.  International trips are higher, although most airlines still do not charge for (at least) the first bag for most international trips.

The fees are available on their airlines websites.  eg, for both [United Airlines][1] and [American Airlines][2] it's $25 for the first checked bag, and $35 for a second checked bag for domestic flights, and whilst the first bag is free for international flights the second will cost you up to $100.  

[Virgin America][3] depends on the type of ticket you have purchased, but is generally $25 for each of the first two check bags.

The main exception to the above is [Southwest Airlines][4], who do not charge anything to check your first 2 bags (with standard weight limits/etc).

Some airlines, such as [Spirit][5], charge different rates depending on whether you pay for your checked bags in advance ($28/$35 for domestic up to 40 pound/18kg) or at the airport ($38/$45).  Mind you, Spirit also charge you between $20 and $45 (up to $100 starting in November!) for your carry-on bag, so avoiding them completely is generally the best option. 


  [1]: http://www.united.com/CMS/en-US/travel/Pages/CheckedBaggage.aspx
  [2]: http://www.aa.com/i18n/travelInformation/baggage/baggageAllowance.jsp
  [3]: http://www.virginamerica.com/vx/fees/
  [4]: http://www.southwest.com/html/customer-service/baggage/checked-bags-pol.html
  [5]: http://www.spirit.com/optionalfees.aspx