Casset is a new asset management library for Ruby, and will support Sinatra initially.
It's a port (with more) of fuelphp-casset.
Its main feature is advanced asset grouping, and it also suppots parsing (SASS, etc), and minifying.
This project is still pre-release work-in-progress, and isn't yet advanced enough for use on a website. It will be up to v1.0 soon, though!