NOTE: Some have commented on how large the hole is. I replaced a broken post, whoever installed it dug a hole this large, and filled it with 1 foot of concrete. It took 3 hours to jackhammer it & remove it, RIDICULOUS!!!
Recently, I replaced a fencepost without using concrete. I tried a new product made by Sika called Post Fix, a two-part, pre-proportioned polyurethane resin which when mixed produces an expanding foam. Each package will work with an 8" diameter, 3 foot tall Sonotube. I am very satisfied with this product, it works great! The best part is if the post rots again in 25 years, the next homeowner will have a much easier time removing it, than I did.
As of June 10, 2016, this product is on sale at Home Depot Canada for $15.75 per bag. Reg. price is $16.70