Share 'Famously Skilltesting Cards'

Inspired by: http://www.channelfireball.com/articles/legacy-weapon-therapy-session/     * Cabal Therapy     * Brainstorm     * Fact or Fiction (and thus, I guess, Sphinx of Uthuun, kinda)     * Gifts Ungiven What other cards are famously skill testing?  Conundrum Sphinx when you both have one?  Force of Will?