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. Marguerite, the About.com guide to Pilates, describes more reasons why Pilates is so great for cross-training. And she also has a lot of excellent Pilates workouts. Here's just a sample: