What are some real-world applications of Red-Black trees today?

http://docs.oracle.com/javase/7/docs/api/java/util/TreeMap.html uses Red-Black tree as its backend. There are two popular Balanced Binary Search Tree: AVL Tree and Red-Black Tree. Both offers O(lg n) search time. But the hidden constant behind Big O makes AVL Tree more suitable for search and Red-Black Tree for insertion-deletion. Insertion-deletion takes less time in Red-Black Tree than AVL Tree. That's Red-Black Tree are more popular than AVL Tree although implementing Red-Black is very complicated task.

The process scheduler in Linux uses Red Black Trees. The red black trees are a replacement for run queues which had priorities for processes on the queue for the scheduler to pick up from. The Completely Fair Scheduler (CFS) is the name of a process scheduler which was merged into the 2.6.23 release of the Linux kernel. It handles CPU resource allocation for executing processes, and aims to maximize overall CPU utilization while also maximizing interactive performance.

Red Black trees are used in http://en.wikipedia.org/wiki/Completely_Fair_Scheduler in Linux Kernel Computational Geometry Data structures In various implementations of Associative Data structures (for e.g C++ STL uses RB tree internally to implement Set and Map)

While this doesn't directly answer your question, but the C++ STL implementation of Map uses a Red-Black Tree.

Red-black trees are needed when height balancing is a desirable trait, and insertion/deletion must not be too slow either. RB trees are common in the Linux kernel. Two of the places where you can see it are: In the http://en.wikipedia.org/wiki/Completely_Fair_Scheduler implementation - the process virtual runtimes serve as the key. To keep track of the virtual memory segments for a process - the start address of the range serves as the key. As you may notice, in both cases it is important to retrieve the node by their key rather quickly from performance considerations.