1. Looking for a job in the near future? Say so!
I noticed a few posters with small flyers tacked next to them that said "I'm looking for a postdoc." The best ones even had a small folder with business cards or copies of their CV.
2. International conference? Make a small, translated version of your poster.
I saw this at the poster of Kayce Bell -- she offered 8.5x11" copies of her poster in either English or Portuguese. It probably took her a while, but it was a nice touch!
3. Phone scancodes.
One poster was accompanied by QR codes so that people could download a copy of the poster to their phone or view their presentation slides. Cool! I can think of other uses too -- People could scan the QR code to go to your website, download your CV, see the published paper, etc...