I quite like the Varnish Custom Statistics, the idea behind it is super simple (aggregate data about classes of requests) and yet, its use cases are extremely diverse: people use it to monitor most requested URLs, to watch for brewing backend issues, to do A/B testing, or to create image walls to show the most read articles (here's the article about how it's done).

We just love it when our customers use our products in innovative ways that really help them create business value. One of the most recent examples of such use is how our customer vg.no. (Norway’s most popular internet news provider) is using Varnish Tools in combination with D3 to visualize their most popular online content.