All of the major ones. This is a common practice. Look for ones that don't charge drop fees, like Alamo via Costco.
We did this last week with a Budget rental from Hilo to Kona, and as SheaG mentions, all the major rental agencies offer it. When you do the booking on-line, look for a checkbox that says "return to a different location".
There will be an additional drop charge - I think ours was about $50. We are not Costco members (mainly because there is a Sam's Club about a mile from our house), so it was actually cheaper for us to pay the drop charge directly than pay for a Costco membership to eliminate it. Plus I preferred to go with Budget since I am a Fastbreak member with them and usually get pretty good rates (plus bonus airline miles).
Be aware that the drop charges vary from agency to agency, and may even be cheaper going one direction vs another since they are based on supply and demand. Driving a car from a "low demand" area to a "high demand" area is typically cheaper than doing it the other way around.
Your best bet is to price out as many agencies as you can and book the cheapest rental you can find ASAP. Websites like Travelocity.com and Kayak.com are good for getting a feel for pricing. Then continue to monitor the rates and rebook as necessary if they go down - agencies are always running unannounced specials and often it's possible to get a better price if you keep an eye out. Also check with the discount sites - we booked a rental on Oahu through DiscountHawaiiCarRental.com and got a much better rate than we could do on our own.
Best of luck with your rental and enjoy your trip - ours was a blast.
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