Well let me add this to the list:
Micro Advantage QuickiDrive QD64
http://www.microadvantage.net/consumer/index.php?class=5&product_id=13
There's one on eBay right now for 20 bucks:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=5144863251&category=51073
I can't see any reason why the 128MB and 256MB versions won't work too.
I also just tried an old "SIIG US2260 Smart Media Card Reader" with a 128MB Sandisk card in it and that worked too.
It might be worth listing what age of PS2 you tested with (in my case
a brand new SCPH-70000 from the Racing Pack .. man it's tiny!)
I did try hooking up a digital camera (a Fuji A330 in case anyone cares) to the PS2 via USB cable and that did not work.
Attached, one from each working device. The Sauber pic from my S16 license test pass run ("ohhh yeahhh" indeed) and the other is the Trial Mountain monkey (he lives!)
For the person that asked how it worked in practice, it's like this:
1. Jam the USB drive in any USB slot at any time
2. When you're happy with the photo, choose the USB option and save
3. If the progress bar goes all the way across, it worked. If the progress bar hangs (for a very long time) about 1/3 of the way, it probably isn't going to work. It always stops for a little while, but when I tried the camera it just sat there - I had to reset in the end.