I'm an avid user of Basecamp, but had never heard of ProofHQ until I read this 37signals blog post: ProofHQ and Basecamp: Offer your project team richer review and approval tools. It seems ProofHQ is a "web-based design collaboration, proofing and approval tool for brands, agencies, designers, print and production" which can be integrated with Basecamp. Cool!

I just signed up for a free account with ProofHQ and am going to attempt an integration.