I thought it was high time we had a topic for this!
Anyway, Vocaloid is a pretty popular vocal synthesiser with which one can make songs. Each voice has a character assigned to it, and there are both male and female official Vocaloids, as well as fan-made ones created by adjusting the pitch on them or something.
There are quite a few Vocaloid songs uploaded from niconico douga to youtube, some with English translations of the lyrics accompanying them. If you don't know about them, and are curious, simply running a search there should give you plenty of songs to listen to.
There are also covers of the songs by actual people around, too.
Personally, I like Kaito the best out of the Vocaloids! His voice can sound very artificial, even more so than some of the others, depending on how he's used, but I think that's interesting.
I'm currently liking
this cover of Magnet, which is actually surprisingly good.
I'm lookiong for some song recommendations, too, since I've been a bit out of the loop recently. I'd be much obliged to read yours!