A brand of Canadian beers, produced at their eponymous brewery in Guelph, Ontario.
They produce several different brews - including the ever-popular Honey Brown lager, Cream Ale, dark ale, Silver Creek lager, and steam beer.
As a company, Sleeman's has now reached the mid-level stage, in that they're too big to be considered a microbrewery (and they have also acquired a few other small Canadian breweries, including Upper Canada), but haven't reached the "mega" brewery size, ala Molson and Labatt's.
Their official website is at http://www.sleeman.ca.