What is a good collection of alcohol to keep around for someone who doesn't drink? I don't drink alcohol at all, but I'd like to keep some kind of selection around to offer to guests who do. I'm not looking to fully stock a bar, but at least a few different options that can accommodate a variety of tastes and also not require prompt consumption.
The kinds of alcohol you'd want can differ depending on the occasion or type of hosting you'll be doing. If a friend helps you move a couch, for example, it would not be unexpected to offer them a beer afterward as a thank you. 

Wine can be offered but is probably more acceptable in the evenings or with a specific meal. One bottle of white and one of red should be sufficient. Chardonnay is probably a safe bet for white wine and a Merlot or Cabernet Sauvignon for red, but your guests' individual tastes can obviously differ.

Liquor like vodka or whiskey can be used to make a variety of cocktails (again, during or after dinner are commonplace) and for guests who want to party these can easily be used for shots as well. Many people have preferences towards specific brands, but if you buy a $20-30 bottle you won't have many guests turn it down just over the name.