Windows Presentation Foundation (WPF) has recently had a refresh to its performance tools. You can read more about it on the What's New for Performance Profiling Tools for WPF post.
The suite of tools includes a bunch of very interesting tools, one of which is a string allocator profiler. The good news is - you don't need to be working on a WPF application to be able to use it! You can just point it to any app you have, and you'll get a tree view of function calls and the count and size of strings allocated - very handy stuff to find where those allocation are coming from.