This is a non-exhaustive list of prototyping and mockups apps.
Balsamiq
Commercial, 30-day trial version
Web-based.
InVision
https://www.invisionapp.com/home
Marvelapp
Freemium.
Web-based.
Moqups
Freemium.
Web-based.
Pencil Project
Open source, free.
Desktop-based. There is a discontinued web-based version (Mozilla add-on).
You can check this post about Evolus Pencil.
Sketch
Zeplin