myBalsamiq is by far the most simple/cost-effective tool I've used for wireframing. Being distributed with version control was particularly unique and helpful, as far as potential competitors go.
As a random usability thing, they also handle renewal notifications without being annoying. I don't need an email every hour that my subscription is expiring, and the earth doesn't shatter once it does. If I decide not to renew, I can still download all the mockups I made when I had the subscription. If I decide I need to get back into the program, a couple clicks and I'm back in for $12/mo. Small stuff like that makes feel like they're not trying to suck every penny out of me, which is why I don't hesitate to go back to them when I need to.
As a random usability thing, they also handle renewal notifications without being annoying. I don't need an email every hour that my subscription is expiring, and the earth doesn't shatter once it does. If I decide not to renew, I can still download all the mockups I made when I had the subscription. If I decide I need to get back into the program, a couple clicks and I'm back in for $12/mo. Small stuff like that makes feel like they're not trying to suck every penny out of me, which is why I don't hesitate to go back to them when I need to.