Valiant Comics are currently having a big linewide sale on Comixology and over 100 comics are free.
They are mainly #1's of stories featuring characters like Archer & Armstrong, Bloodshot, Britannia, Doctor Tomorrow, Eternal Warrior, Faith, Geomancer, Imperium, Ivar: Timewalker, Livewire, Ninja-K, Psi-Lords, Quantum & Woody, Rai, Savage, Shadowman, Doctor Mirage, The Visitor, War Mother and X-O Manowar.
Also unlike some other comics publishers Valiant allow you to download their comics in CBZ or PDF format in the "backups" section of your account to keep permanently.
Valiant have also been giving away a free comic (issues and volumes) each weekday on their Twitter page which I made a thread for and thanks to alvian for the links and continually updating that thread:
Thanks OP! Added 112 free comics to my library :D