If I recall correctly, Skylight hooks into the ruby interpreter to do real time performance monitoring for Rails apps. They felt they couldn't get the performance they needed out of pure ruby, and felt that writing their hooks in Rust would be safer than in C. You can see Yehuda's presentation here. It's about an hour and 10 minutes in:
Tilde, the company started by Yehuda Katz and Tom Dale of Ember fame, is building a product called Skylight. You can roughly think of it as a New Relic competitor.
It uses a Ruby extension written in Rust to collect the performance data.
