To me, the best ongoing ideas that came to fruition as a result of FT participation are:

MileageAddicts Mileage Workshop.
Let's say you are a relative newbie. You think you've more or less maxed out your earing opportunities, but you're too lazy or have too slow a connection to do a thorough search for bonus offers. Well, if you haven't been to http://www.mileageworkshop.com/
then you haven't seen nuthin' yet. A true masterstroke first announced at http://www.flyertalk.com/forum/Forum1/HTML/000045.html

Premex's Unofficial United Elite Super Happy Fun Time.
Considering achieving elite status on UA? Better visit http://members.aol.com/millionmileflyer/uahelpdesk.html
First announced at http://www.flyertalk.com/forum/Forum50/HTML/001598.html

I put Blondebomber's *A comparison chart on the same level, but since I'm no big fan of *, I'll leave finding that chart up to the reader (or some subsequent poster...).

My favorite "social" memories go back to when Merry Flyer had more time for us and used it on battles of acerbic wit and folly (mostly the latter ). But always you ended up smiling

My most embarassing FT moment: Believing completely that pjaucutt was who he said he was and even allowing him to make some reservatios for me in Paris...