I really like the Phoenix, a small bluetooth speaker from Beacon Audio. They sent me an evaluation unit a couple of weeks ago. It paired very easily to my iPhone (unlike a lot of bluetooth devices, which are pains-in-the-neck to pair) and the sounds is loud and clear, given its size. It's a vast improvement over the iPhone's built-in speakers. Because it's so small, I find myself using it quite a bit when I'm doing work outside. It's also great for playing games and watching videos. Not sure about the lithium battery life yet, but you can recharge it with the included USB cable. I find that I use it a lot more than the Jambox, which feels too precious to take out. It's available directly from Beacon for $99. UPDATE: A couple of folks in the comments have said that the Satechi ST-66BTA ($39.99 on Amazon) is very similar, if not nearly identical.
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. Come and hear Mark speak at the ALA conference in Chicago on July 1.