What are some alternatives?
When comparing Terrastruct and Whimsical Wireframes, you can also consider the following products
Plectica - Easily diagram anything, together
Method Wireframe Kit - Find your method of being incredibly fast in prototyping.
SnapUp - Wireframe faster / create amazing user interfaces
Mydraft.cc - 100% free wireframing
UserGuiding - UserGuiding - Better User Onboarding Without Coding
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.