Fitocracy

This is by far the best way I've found to stay motivated to work out. The interface makes logging exercise easy, and also gives me the ability to plan ahead, lay out different exercise programs and even get existing programs by professional trainers. I haven't tried those, personally, but they look good.

It's also a social networking site, so you can get inspiration from a huge community of Fitocrats. There are groups for various interests, much like on Facebook, but most are exercise related.

Highly recommended for anybody who likes to exercise and wants motivation. Or just a fun way to track their workouts.