Some of my favourites over the years:
"I live with...", where we ended up making photoshops of MP for "Freedom...serenity...kindness..." from The Shattered Fortress.
"Mouse", of course!
Even though it wasn't completed, "Scenes from My Memory" was pretty awesome.
And in more recent times, both the cone and Whahíbrido pickingant discussion threads have had me in stitches.
I love how these moments just happen. The forum takes something and just runs with it, with hilarious consequences. Good place to be, this.