http://boardingarea.com/blogs/viewfr...es-per-dollar/ and http://boardingarea.com/blogs/tmtrav...a-card-holder/ mention that the Fidelity Investment Rewards Visa and American Express now offer a redemption option to Star Alliance member Air Canada's Aeroplan miles that works out to 2 miles/$ for the AmEx and 1.5 mile/$ for the Visa (and 2 mile/$ after $15k/year). This is basically the best non-promotional mileage redemption I have heard of, and works out to the following cost per mile for paying US federal income taxes on line:

officialpayments.com or pay1040.com with American Express:
2.35% fee / (2.0 mile/$) = 1.175 cent/mile

payusatax.com with Visa:
less than $15k/year spent: 1.95% fee / (1.5 mile/$) = 1.3 cent/mile
more than $15k/year spent: 1.95% fee / (2.0 mile/$) = 0.975 cent/mile
[Edit 9/23/2010: Thank you Ikar for pointing out that payusatax now rejects Visa credit cards. I verified that this is specifically the case with my Fidelity Visa.]