Bookmark It - Create awesome notes on YouTube videos
LibHunt - LibHunt tracks mentions of software libraries on relevant social networks. Based on that data, you can find the most popular projects and their alternatives.
Bookmark OS - Bookmark OS is like Mac or Windows optimized for bookmarks.
Ruby Toolbox - Explore and compare open source Ruby libraries
Siimpl Bookmarking - Start collecting and organizing what interests you
GitNews Web - Trending repositories from GitHub, HackerNews & Reddit