Probably 70% of my brews are session beers that finish up around 4%. One of my favorite brews is Northern Brewer's Mild Ale kit. It usually comes in around 3.8%, but is dark in color, and lots of flavor. I highly reccomend it.
Many people are recommending Mild and Ordinary Bitter. I'd like to take this opportunity to add my +1,268,307.
These styles get short shrift these days, amongst all the Arrogant Bastards and Belgian IIPAs and stuff like that. Like others have admitted, I like to drink beer. Lots of beer. I don't like getting fit-shaced, and hangovers are undignified.
Thus I brew and drink an awful lot of "session beers". In fact, I try to keep at least one available at all times; I brew Mild from September through March and Bitter the rest of the time.
Mild and Bitter are amongst the most simple beers on Earth to brew. Two or three grain ingredients, one variety of hops (so you can buy them by the pound), and uncomplicated fermentation and packaging. What more could you need?
Here's a nasty little secret I started doing about a year ago. I've been on a quest to simplify and streamline my entire brewery, including the recipes. So I tweaked everything, and you know what? My beer has been getting better and better. My current Bitter recipe is pale malt and 10% 55L crystal, with 1 oz of Fuggle to bitter and 1 oz at flameout. My Mild recipe? Add 6 oz of chocolate malt and drop the flameout hops. What could be simpler?
Enjoy lower-octane beers!
Bob