I believe Amnesiac is a collection of Kid A songs that didn't make the cut. I consider them b-sides, although it is an official album release.
Speaking of b-sides, I forgot about True Love Waits. Jesus, Radiohead has the market cornered on epic b-sides. Talk Show Host, True Love Waits, You and Whose Army?, Permanent Daylight, I Might Be Wrong, Like Spinning Plates, Palo Alto. Not fucking bad.
For the veterans here: I understand these are "true" b-sides in the sense, but as not being included on official album releases, I refer to them as such for this purpose.