Any of the major car rental companies will allow a "One-way" rentals. Occasionally there are restrictions around specific locations, but in general if you're planning to go from a major airport to another major airport you'll have no problems at all.
The issue is the price, which can vary dramatically depending on a number of factors. Some rental companies will charge a "drop fee" which will be a fixed amount, sometimes based on the distance between the two locations. Others will charge a higher per-day rate for one-way rentals. Some others will not charge extra for the rental, but will charge you an additional fee based on the number of miles you travel on the trip (ie, the actual miles you drive, not the straight-line distance between the two locations).
The best option is to look around at all of the various rental companies for the specific city pair and dates you're after - if possible in both directions as sometimes it's cheaper in one direction than the other. Travel websites like Orbitz.com can be good as they will show you multiple companies in one search, but sometimes for one-way you'll get better rates when searching directly on the rental car companies site.
Occasionally the rental companies will run "deals" for one-way rentals when they have a need to get cars out of a location - a few years ago I did a 3-day rental from Portland, OR to San Francisco, CA for a total cost of less than $20! These details are often listed on the rental car websites, but be careful as they do often have some strict restrictions such as no more than 7 days total rental. eg, Hertz currently has a deal for pickup in Arizona and drop-off in California, Colorado, Kansas, New Mexico, Oklahoma, Utah or Texas for $5/day!