The first volume of ボーイズ・ラン・ザ・ライオット is free to keep right now on bookwalker.
I haven’t read it yet, but I’ve seen it mentioned on the forum recently (e.g. here), in a way that made me very much want to read it, so I thought I’d mention it in case that was true for other people too!
There’s also a handful of other free stuff related to the manga periodical ヤングマガジン, but I don’t see anything that pops out as something I’ve read or heard about, so I can’t vouch for any of it yet.
Also I didn’t notice since I already have it (but haven’t read it yet…), but a manga often mentioned as a good beginner starting place, Flying Witch, has a couple volumes free right now as well.