BrewDog are currently doing a nice little deal on 4 cans of their "carbon negative" lager called Lost, you pay £1.95 postage and the cans are free! And no, it's not a scam!
Two downsides:
1. It'll take you 10-60 minutes to get onto their website unsurprisingly (don't close your browser!)
2. They're not actually despatching it until the start of February (I think)
(3. it's a lager and not an ale!)
Also Beavertown Brewery seem to be doing some interesting stuff, could also be worth a look, my local Sainsburys stocks a few of them.
Enjoy!
Phil