I've been on the JBL Micro/Clip (non-bluetooth) for ages and it's pretty great... small, focused sound, surprisingly punchy. Not too expensive. Ill Buddha rocks a massive battery-powered amp tho. He's mean.
My favourite has been and always will be the Nixon Blaster. Just bought a mini blaster specifically for the portable set up. I'm trying to keep my stuff managable and truly "portable", though it's getting hard with all this cool gear to want to aquire!
I recently picked up a JBL Clip2 from an outlet store. It has a carabeener clip and a 1/8 inch cord hard wired. Its small and seems to work well for personal needs. I have a couple other blue tooth type speakers as well fr Jambox and JBL that I've had for a long time.