What are some alternatives?
When comparing Terrastruct and Wireframe.cc, you can also consider the following products
SnapUp - Wireframe faster / create amazing user interfaces
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
Plectica - Easily diagram anything, together
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
OmniGraffle - OmniGraffle is for creating precise graphics like website wireframes, an electrical system designs, or mapping out software class.