Kanken Journal is a free magazine published by the people who run the Kanji Kentei (the official kanji proficiency exam designed to be taken by native Japanese).
Each issue includes
-Japanese grammar/kanji usage articles
-Sample questions for every level of the Kanken
Great reading practice with kanji exercises at the same time!
So far my favorite article was an explanation of the ～たり grammar point and how its misuse can lead to confusion. (vol. 19)