Many runners know about the benefits of cross-training, but they always want to know: What's the best form of cross-training for runners? I don't think there's an obvious answer to this question. Any form of exercise that works your non-running muscles and gives your running muscles a bit of a break can be a good cross-training activity for runners. The most important thing about the cross-training activity is that you enjoy it, so you'll actually do it. For example, I could tell you that swimming is great cross-training for runners, but if you absolutely hate the thought of doing laps in a pool, you're not going to suddenly work swimming into your schedule.
That being said, I do think that Pilates is excellent cross-training for runners. The Pilates method is a form of exercise that emphasizes core strength and flexibility, important elements for running that are often ignored by runners. Here are some reasons why Pilates is so great for cross-training. And check out these excellent Pilates workouts. Here's just a sample: