What Bumble said is pretty standard, but I have to say - actually GOOD pens do tend to stay around.
What Zappit said is funny and easy, may get you remembered for a while but won't leave any long-term impression, of course - once folded, the appeal is gone.
Depending on the type of people you want to appeal to, something I find gets used a lot and sticks around forever, is...err, don't know the english word. Sort of a saucer with holes in it, you can put those typical plastic cups in from coffee machines. Inflatable (for beer on festivals) or slightly sturdier (for coffee in office environments), both see a lot of use - and we constantly run out.