1. The idea that Snape carries at least one black handkerchief, suitable for offhandedly dispensing to weeping females (who then keep the handkerchief for their own use, instead of washing and returning it as any responsible young woman would do).
2. Exaudio. Apparently this charm, which allows the caster to subvocally communicate with the recipient without being overheard, is even handier than I thought it was, because a few different authors now have seen fit to borrow it.
Not bad for a few months' work. ;)