USB devices that work:
See HowTo.SlugAsAudioPlayer page for info about audio devices.
Chances are that the NSLU2 thinks of your MP3 player and your digital camera as a hard disk or a flash memory stick. So you might try them out on port 2 and follow the HowTo.AttachAFAT32FormattedDrive.
Note: The information below is outdated and holds for Unslung 3.x and earier only. Starting with Unslung 4, the drivers for CD/DVD support are available.
USB devices that don't work on the NSLU2 using Unslung versions prior to 4.x:
The reason why those don't work is as follows: the Linux kernel that was shipped with the NSLU2 does not open up for adding modules supporting such devices (most modules will have to rewritten to work). The Unslung firmware only does very little modifications to the standard firmware - but it definitely does not replace the kernel to add the support. Apart from the hacks Sercomm did to the USB MSC driver, everything else is standard except it's just not compiled in. Nothing else was changed or crippled.
Don't turn away:
The OpenEmbedded builds will hopefully open up the possibilities in the future. If you want to get involved ... read Main.HowToGetInvolved.
But if you don't care for the original Linksys functionality and just want a little linux box with full USB support, then OpenSlug is for you!! Join the fun today and help out! (It's not what OpenSlug can do for you, but what you can do for OpenSlug!!) See you on IRC!