CodeTriage - Help out your favorite open source projects and become a better developer while doing it.
Mergefly - A modern UI for GitHub pull requests
24 Pull Requests - 24 Pull Requests is a little project to promote open source collaboration during December.
PullRequest.com - Code review as a service
Gitpay - Add bounties to solve Git issues from projects.
Codementor - “Instant help from expert developers. Codementor is your instant 1:1 expert mentor helping you in real time.”