As I understand, Ruby Toolbox site has laid down for more than six months. It's really sad, but may there be any similar resource?
In rubygems, I cannot select the subject or the category of a gem; only little description is available.
I wrote a gem, and want to distribute it.
Checkout Awesome Ruby, it bills itself as:
A categorized community-driven collection of awesome Ruby libraries, tools, frameworks and software. The essential Ruby to build modern Apps and Web Apps.
Though there's no guarantee that your gem will be added. Checkout their contribution guidelines.
For similar lists for other languages checkout the list of awesome lists.