Ton

Meaning: in cricket, a hundred runs; also known as a century. The OED has this as definition 5a.(colloq*), but wiktionary recognises that cricket is its own special linguistic category, and describes this sense as 7. (cricket). May or may not have been borrowed from darts, where a ton means a hundred points.

Usefulness: 2 (its usefulness is variable - for example, Australia did not get to use it today**. Can be extended figuratively to a hundred of anything, such as blog posts: "This post brings up the ton for Six Degrees of Sir Thomas.")

Logofascination: 1 (Words often reveal our assumptions or biases; I have a metrical bias*** - since a tonne is 1000kg, I wondered how a ton could be 100 of anything. All the senses of ton - weight, lots of anything, a hundred of anything - are originally from the word we now know as tun, a very old word for a very large cask. The cricket term is possibly a specialised application of the sense 'lots of anything'.)

Cricket

Meaning: My favourite thing, but if you'd like a longer definition, the online Oxford says:

an open-air game played on a large grass field with ball, bats, and two wickets, between teams of eleven players, the object of the game being to score more runs than the opposition

Usefulness: 1 (if you talk about it as much as I do. I'm rather impressed I've done 77 posts without mentioning it here.)

Logofascination: 2 (etymology unknown, and about as many theories as people to ask: the Inky Fool talks about a possible connection to stool-ball, wiktionary features someone else's theory about Dutch origins, and there's a bunch more theories over at wikipedia.)