Welcome to Thinky Third Thursday, curated by Alan Hazelden of Draknek & Friends - a monthly roundup of thinky puzzle games that have caught my eye.
Often I'm using this newsletter to highlight commercial games that I think are worth buying, but I don't want to neglect the many great games available for the low price of free. This past month I've played several web games that impressed me, so I wanted to put the spotlight on a few that particularly stood out.
Recent free webgame highlights:

CO-DOOR, by yuji_ap
This is short and sweet - just four floors of puzzles, each containing a mysterious black door connecting to a featureless void. If you only have 15-30 minutes to spare, this is the one to check out.
Not that my recc should mean anything when Alan Freaking Hazelden is the OP, but totally seconding the plug for Hempuli's "Covemount-like" series of small (surprisingly large!) block pushing webgames. Each one is a perfect and surprisingly deep investigation of an absolutely wild mechanic.