-John Maynard Keynes
This is based partly on Edge's ridiculously interesting annual question (and accompanying book) for 2008: What have you changed your mind about? The link leads to a bunch of answers by a bunch of scientists, a couple of philosophers, and Brian Eno. The subtle difference is that a lot of these guys changed their mind as a lot of their own individual research conflicted with a closely- held theory, whereas here we're mostly talking about ideas other people have sold you on.
Of course, if you're in a position where you've done original research that disproved something you used to believe, that's awesome, and definitely tell us about it.
DON'T talk about things that gave you an opinion on a topic you never had a firm opinion on beforehand (e.g. Tim Flannery's "The Weather Makers" proved to me that climate change is real, when before I wasn't really sure what I thought about it; I never knew how great free trade was until I read Thomas Friedman's "The World is Flat"),
DON'T go for stuff that sold you on something you already suspected (e.g. "Richard Dworkins' "The God Delusion" made me become a much stronger athiest than I used to be), and obviously
DON'T talk about stuff you find really persuasive if it hasn't changed your mind (e.g. "I still believe in gun control, but Lott's "More Guns, Less Crime" is a REALLY convincing counter- argument"; "even if my parents hadn't brought me up as a vegetarian, reading Peter Singer's "Animal Liberation" would have turned me into a vegetarian anyway.)
Discussion about what other people say is completely fine, but keep it civil, keep it fairly brief, and move it to a new thread if it starts dominating the discussion here.