These sales are usually temporary, so don't miss out when you see one posted! If you post a sale, be sure to link to it if possible and state for how long it's up if you can.
Most big game sales and freebies are posted on this subreddit, so be sure to check there and post any you think people should know about in this thread. Epic Games also gives out a free game every week if you use their launcher. Check out whatever is free now at this link.