The human Y, that is. The Science Creative Quarterly has a very thorough (ie, make sure you have some time to spare) review of the mammalian Y chromosome (focusing on the human Y). The article covers the origin and evolution of the mammalian Y and what the degeneration of the Y means for the future of human male fertility and sex determination. I should point out that the mammalian Y chromosome is an anomaly in origin and sex determination. In fact, every single sex determination system and sex chromosome system that I know of differs from all of the others in some manner. It looks like I’m going to have to write an entry on the evolutionary genetics of sex determination in everything other than mammals (because the SCQ has already covered the furry guys).
Added 20March2006: Of course, eutherian sex chromosomes are boring compared to monotremes. Check out the crazy stuff going on in the platypus.