What do you mean by "use" for penny stocks? If you mean which broker, it's Royal Bank Action Direct in Canada. Commissions are generally higher for penny stocks (because you pay commission on the number of shares at that level) but that's only if you have enough money to break the minimum threshold which is, for them, I believe $29. I don't have nearly enough money to break that minimum threshold.
I know the definition of penny stocks is generally anything from $0 to $5, but most of the time I'm dealing with stuff that is under $1.00. It's a very risky game, yes, and not appropriate for most people, in my opinion.