David Letterman -- Happy Retirement! Enjoy Free Porn For Life

David Letterman has a shot at a major happy ending when his run at the 'Late Show' comes to an end Wednesday night -- he could snatch a lifetime supply of free porn ... if he's up for it.

The porn site GameLink sent the late night legend a letter offering him free access to its vast online library of adult movies. The letter even highlights a title to get Dave started -- “F*** My Mom & Me #9" ... which famously (to pervs) spoofed Dave's relationship with Bristol and Sarah Palin.

GameLink says it's merely thanking Letterman for the many years of laughs and entertainment he's provided viewers.

We doubt Dave will take 'em up on it, but it beats anything AARP has to offer.