Sleep in spite of obnoxious roomates, or study in total silence! You don't want to get hooked on these, but in a pinch they will make a difference. (via target.com)

i love these earplugs
(though my personal preference is for the lovely sea-green colored ones, rather than the pinkish ones pictured).
they are soft and comfortable, conforming to the contours of your own ear quite nicely. they do a fine job of blocking noise.
the fact that they are reasonably priced and come multiple to a box means that you can stash a pair on your bedside, in your travel kit or purse (perfect for having should a nightclub have an overwhelming sound system), in your desk and so on.
plus, with the generous box you will have replacements on hand should you lose one, or for for hygiene's sake need to replace any.
your sense of hearing is worth protecting!