Travelling minstrels, as in bards, actors, etc... It's used in oh so many plays and movies (anybody see Rosencrantz & Guildenstern Are Dead? (bad spelling I know) ).
Bandits. Used a lot I know, but you could make them original (or at least interesting). How about a Robin Hood type band... Steal from the rich, give to the poor, PCs are often rich :) :). This is especially good if the PCs are all (supposedly) good and claim to support this kind of rampant do-goodery. Just wait till they realize they are now the targets :)
Monks or Pilgrims. These people can be nicely annoying to PCs. They may be overfriendly, or attempt to convert characters, or ask for donations (travelling Hare Krishnas anyone? :) ). Monks can also be really useful... they know all kinds of stuff, or, if you're into that kind of thing, they could be carrying a holy relic around with them.
Tax collectors. At certain times of the year the Lord/Sheriff of a barony would send people around to all the villages to collect the king's taxes. These people would be heavily armed and would carry a whole pile of money. The PC's might be tempted to turn bandit- even if they had good intentions. However, stealing taxes is one of the best ways to incur the wrath of the local lord, because then he must pay the king out of his own pocket!
The single, unassuming stranger. Who is he? He could be a wizard in disguise... he could be a dishonoured knight... he could be a young man looking for adventure and escaping a nasty past. Why not just have someone ask to accompany the characters. Make the person act mysterious and listen to the players speculate. Players have really good ideas sometimes :) (If they decide to kill him for being mysterious then (a) your players are bloodthirsty and (b) this guy could be a totally innocent traveller, perhaps with powerful friends).