http://zilch.playr.co.uk/zilch.php
I don’t often thumb-up Flash games – in fact, I don’t often play Flash games – but this is one that I learned IRL at a cottage years ago, and have enjoyed ever since. Trying to find others to play with me, though, is hindered by my complete inability to lucidly explain rules or gameplay. As far as I’m concerned, this website is an excellent primer.