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If you, like me, enjoy skyscrapers and infrastructure being built, I highly recommend the Skyscrapercity forums. There's active discussions on nearly every big city in the world. Literally dozens of millions of posts there. I enjoy designing transport maps so this Transit Maps tumblr is really cool. The guy reviews tons of new and old metro / rail maps and actually knows his shit, which is a rarity. Besides that I have some really specific forums I irregularily check, most which have something to do with either urban planning or rollercoasters (a hobby of mine).