Add-ins and add-ons

Hi all,

I'd like to put together a list of useful plug-ins and add-ons (both
software and physical) that work with ODK (I'm thinking GPS apps, readers
etc). I'm more than happy to make this a public resource on a blog or
something. I'd be really grateful for any suggestions for components to
include - perhaps ones you've worked with, or are working with.

Thanks,

Calum