Here's a list of some suggestions on good beers many have offered.
A great first beer to try is a Williams favorite: Milwaulkee's Best aka "The Beast". Excellent, full flavor--it's a real beer experience. Yet light enough to play Beirut or Flipcup with. Also very well priced for such a great beer.
Or try one of the Belgian offers that are usually available at Cole ave. and even spring street:
Hoegaarden Witbier (white beer) 5%alc.
Blanche de Bruges (white beer)
Duvel (golden ale, absolutely amazing) 10%alc.
Grimbergen Tripple (abbey/trappist ale)
Lindeman's Lambic ale (flavored with cherries, peaches, etc.)
hoegaarden > blanche de bruges. duvel and grimbergen are tasty. disagree about lindemans, its pretty much a malt beverage. for a nice selection of belgians, try the beverage den on rt 7 in bennington. they've got orval, chimay, rochefort, witkap pater, cantillon gueuze, saison dupont, de ranke xx bitter, and some other stuff. for cheaper belgian style beers, allagash, unibroue, and ommegang are all awesome.
If you hate the Belgians, here are some other great brews (available at Spring st.):
Samuel Smith - the whole range: Oatmeal Stout, Imperial Stout, Taddy Porter, Pale Ale, Nut Brown Ale, Winter Welcome Ale.
Unibroue Le Fin du Monde
Celebrator Doppelbock
Stone Imperial Stout
Otter Creek is absolutely fantastic. I especially recommend the Vermont Lager, but all varieties are very tasty.
BBC brews up some mean beers. Almost anything by them is great, except for the Steel Rail Extra Pale Ale. Support local beer!