Those places where you get sucked in and don't surface for hours.
I usually find I run out of all the normal websites I check regularly by mid-afternoon, so am always pleased when I stumble on something absorbing. The most recent one is basenotes.net, a website for people obsessed with perfumery who call themselves "fragheads" and rank top ten most underrated scents and prefer the earlier stuff of various scent-masters and overuse the word sillage. It's excellent, and I know nothing about scent.
What time sinks do you have?
Also, I do have a question about scents which led to this distraction in the first place.