I'm a paid subscriber of Remember the Milk, but I don't think they do a good job differentiating their paid product from the free product. I subscribed because I felt that $24.95 was a fair price for a superior mobile interface, but there aren't many other benefits for paid customers.

37signals does this differently. Their free products are far more limited, and their paid products offer quite a few more features than their free products. This limits the value to the freeloading customer, but I think this makes for a more sustainable business in the long-run.