I've been using Balsamiq for a while now and I love it. Sure it runs on Air and isn't free, but it's one of the best purchases we've made as a bootstrapped star

I've been using Balsamiq for a while now and I love it. Sure it runs on Air and isn't free, but it's one of the best purchases we've made as a bootstrapped startup. Also, the group that runs Balsamiq is awesome and the CEO -Peldi?- writes a great blog.