units.dat and zoneinfo
It's great to read units.dat (probably /usr/share/units.dat or /usr/share/misc/units.dat on your system), the encyclopedic compendium behind the amazing units program. It's got everything from the kilogram, meter, and second to the wey, sack, sarpler, last, susi, kirat, wukiyeh, periot, blanc, pony, and firkin. Never mind furlongs per fortnight -- that's a snap!
It's especially fun to try combining SI prefixes with ancient and traditional units.
They have even defined loony (you know, the Canadian currency).
This reminds me of the comparably amazing experience of reading the source code for the time zone definition files in the GNU C Library. Take a look at the contents of glibc-2.3.3/timezone, for example. There is so much research represented there, including an attempt to get the time zone name and offset right for every location in the world ever since time zones were introduced there. If you're in a U.S. time zone, for example, try date -d 'now-60years' and look at what you get.
I was taught in school that there were 24 time zones (UTC plus or minus each whole number of hours up to 12). That's what a lot of globes show, anyway, but this is terribly wrong. There are actually many more because of variances in whether and when summer time is observed -- and some time zones have offsets of 0:30 or even 0:15 and 0:45. What's more, if you take a time zone as including the whole history of when summer time was or was not observed in the past (and in which direction, because some Southern Hemisphere jurisdictions observed it in the wrong direction in the past to ease commerce with the Northern Hemisphere but to their detriment in other ways), it seems that there are at least thousands of different time zones. The glibc people have tried to collect and document and substantiate each one.
