An interactive history lesson on the internet. Has loads of webpages, games, and applications to play around with from various time periods. A lot of the work on neal.fun is really cool (you've almost definitely seen his password game floating around) but this one is my favourite.
A webtool that lets you plug in various measurements and compare them to physical objects. Particularly useful when talking to moron americans who think that everyone else uses imperial like they do.
Colour contrast checker. Does what it says on the tin elegantly and responsively. Simpl as.