Anyone have any ideas on small, portable
headphone volume amplifiers?

B&H has a whole bunch of headphone distribution amplifiers from what I've seen in the website...along with some battery powered ones.

Anyone ever tried the Boostaroo or Boostaroo
Revolution?

I've used the Rolls PM50 and I think it's great value for money.
Runs on a single 9V battery
http://www.rolls.com/new/pm50sob.html

External audio meters are great for this. I have a SignVideo meter that has a way better (and more reliable) headphone output than the one on our old GL1. The Beachtek version hooks into the line output and has the added advantage of giving you a second headphone output (from the one on the camera). SoundDevices makes a crazy expensive and probably ear-splittingly loud headphone amplifier that gives you three outputs. I also have a cheapie Radio Shack doohickey that gives you three outputs (and a significant boost) that works well enough. Devices that run off of the headphone output on the camcorder are supposed to work best when the headphone level is set at about halfway and the Radio Shack product doesn't quite give you enough boost for that. The Sign Video amplifier is quite a bit better.

Marco, I didn't know that that it is best to set the
camera level at the midway point when using an
amp.
I believe the Radio Shack device is the same as
the Boosteroo.

At full volume many headphone outputs will distort on loud noises even though the recording is just fine. Keeping the camera headphone volume at one half to two thirds will minimize this.