What games are suitable for a large number of people with only a pack of playing cards?
The best large-group card game that I know of is Up and Down the River, which Wikipedia knows as Oh, Hell. I find the rules given by Wikipedia to be confusing, though, and found a better set here at eHow. (Note that both of these rulesets differ slightly from the version I'm familiar with, as there are endless variants of most of these games.)