This is a solid list for checking backlinks. I've used Ahrefs and Majestic before, they're great for seeing what's already out there. Anyway, a while back someone showed me vladenza.com for SEO link building. Different angle completely. Not a checker tool, but they help get new links from DR20+ sites. Manual placements, no spam, transparent reporting. Just thought I'd mention it since the whole thread is about backlinks and people might be looking for both checking and building options.