For noncommercial websites, https://www.startssl.com/ offers free certificates.
For commercial websites, the cheapest certificates I've seen are PositiveSSL certificates resold by gogetssl [1]. It's $4.55 for one year or $17.25 for 5 years.
Disclaimer: I have no affiliation with either site. I've never used gogetssl, but I will probably give them a shot the next time I need a certificate.
[1] https://www.gogetssl.com/domain-validation/comodo-positive-s...