What are some alternatives?
When comparing Terrastruct and draw.io, you can also consider the following products
Plectica - Easily diagram anything, together
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.
Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
yEd Graph Editor - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
SnapUp - Wireframe faster / create amazing user interfaces
Visio - Simplify and communicate complex information using data-linked diagrams.