I do like the grip top sock one, Dave - that's a new one on me.
The very best tongue-twister I've ever come across is this one (said over and over, faster and faster):
Peggy Babcock
Looks easy...but just try it. When you *have* mastered it, move on to "Peggy Babcock, Babcock Peggy". It's the best warm-up exercise ever for vocal clarity when singing or speaking.