SpeakOut is a prepaid wireless service from 7-Eleven that's quite popular in the RedFlagDeals.com forums. Through August 31 they're offering a $25 bonus top up on the purchase of a $100 wireless top up voucher. And you'll get a free SIM or MicroSIM Card as well!
These offers are both available through August 31 in-store and online. Use this page to find the 7-Eleven nearest you. The prepaid airtime has an expiration date of 365 days from purchase.
Of course, prepaid plans definitely aren't for everyone, but they're ideal for people who don't use their cell phone frequently or don't want to make a three-year contract commitment with a carrier.