Graded readers are a good resources for [extensive reading](https://en.wikipedia.org/wiki/Extensive_reading), at least for people who are not yet sufficiently proficient to read texts written for native speakers of the target language.

We have [graded readers for many languages](https://languagelearning.stackexchange.com/questions/tagged/graded-readers), like for [German](https://languagelearning.stackexchange.com/questions/3214/graded-readers-for-german).

What are good (preferably free) online graded reader resources for English?

As suggested in comments, I looked on ELL, and I found only two suggestions, both closed questions:

* [Tips for easy-to-read unabridged English fiction](https://ell.stackexchange.com/questions/97124/tips-for-easy-to-read-unabridged-english-fiction-for-language-learners) - promoting Wizard of Oz, local public libraries (unlikely, if English is L2) and project Guttenberg
* [Improving reading skills](https://ell.stackexchange.com/questions/63650/improving-reading-skills) - recommending "Start with books for a younger audience" - which are native young speakers, not L2 learners.

So obviously people on ELL know their English, but do not know how to learn it better. We do. :-)