Redis has really solidified itself in today's major content management and social apps. Their most impressive use case is probably over at Pinterest, a site that is considered the third most popular social network next to Facebook and Twitter.
Adam Bloom recently wrote a really interesting case study on the Pivotal blog that explains how Pinterest was able to handle its extreme (1000%+ / year) increases in traffic and keep scaling up its following functionality. As you might expect, Pinterest stores each "follow" on Pinterest in a graph, which is now up to millions of nodes and billions of edges. Here's where Redis comes in:
Ultimately, the team decided to store the entire graph in Redis to serve lots of lists. Immediately, Redis begins to show how it is truly different and acts almost like an in-memory, operational data store.
– Adam Bloom
Go check out the full post on Pivotal (not a long read) and also read the super-fascinating post, "Building a follower model from scratch" on the Pinterest engineering blog.