I typically put mustard on my fries (the unsweetened stone-ground kind or "deli" brown mustard, versus generic yellow mustard, Grey Poupon, or other weird variants).

For a while, I got into malt vinegar, then I discovered that balsamic vinegar is even better. I've also had good results with Wostershire sauce (really, just vinegar plus some seasoning) and various hot sauces (again, just vinegar plus hot pepper bits). There's also a fun sauce called 'balsamic hot sauce' that combines all my favorites all in one place. Yummy on fries.

Probably the most unusual fry topping I've ever enjoyed has been tahini (sesame seed paste). There was this burger joint in Berkeley that was also a middle eastern restaurant, so they had squeeze bottles with ketchup, mustard, and tahini. It's really quite good.

I spent a week in Belgium at one point, so I tried several of the mayonaise variants, but they just don't cut it for me. I had poutine once, in Montreal, and that was unquestionably good, but it's also a heart attack on a plate. It's really heavy in your stomach after you eat it...