Oh wow, just scrolled past a thread about this. Honestly, the 'free' part gets tricky with popular titles. You'll find chapters scattered all over the place, but it's a mess. Aggregator sites pop up overnight, but half the time the translations are clunky or the updates lag weeks behind the official release. I got so fed up waiting for one series that I just caved and got the Shonen Jump app. The subscription is like two bucks a month and you get the latest chapters the same day as Japan.
That said, if you're hunting for something completed or a bit older, some official places do have free rotations. The Manga Plus app by Shueisha lets you read the first and latest three chapters of a bunch of big series for zero cost. It's not the full backlog, but for catching up on current hype or sampling something new, it's a legit way to go without dealing with sketchy pop-up ads.