Not a beer, but a mead. Mead is made with honey instead of grain. I was flipping through Papazian's 'Joy of Homebrewing' and ran across his recipe for this. The very next day, I noticed that there is a cactus patch down the street, and it's prickly pear cactus. I stopped and asked the home owner if I could pick some, and they said I could. I picked about a quarter of a paper grocery sack full, which turned out to be 8 pound of prickly pears.
This recipe made 4 gallons.
16 lbs honey
8 lbs prickly pear cactus fruit. Chop in a food processor until smooth.
Boil fruit with 2.5 gallons water for 2 hours. There are seeds in the fruit, they look odd.
Mix the honey with 1 gallon of water, bring to 170F.
Strain the fruit juice into the honey and water, top off to 4 gallons.
Cool to 70F, pitch wine yeast (beer yeast won't be able to handle the high alcohol content). I used Red Star Premier Cuvee from Wine Corkers (thanks, Nikki!)
Ferment in a corny keg for 1 year. Transfer every month or so for clarity. Samples so far have been excellent, not beer for sure. The flavor is hard to describe, maybe rhubarb wine?
Update: somewhere along the way, Feb 09 maybe, I realized the corny wasn't holding pressure, so I bottled.