Scott Wilson (swguy) is reputed to be the zencart "discount king" so he is probably the best person to ask.

Scott has a wide range of FREE discount modules, and has also developed some commercial ones.

Scott also does private consulting for specific and bespoke needs.

(PS: This is NOT an overt endorsement of Scott - I can however say that I have worked with Scott on several occasions and he has delivered the solutions I required.)