I'm a big fan of Google's webmail system, GMail. Great interface, great reliability, great features. And it's totally free.
The deal is, Google's computers read your emails to work out what sort of things you might be interested in, then present you with relevant advertisements down the right-hand side of your GMail page. That's the idea, at least. But Google also let people pay to advertise to you based on your presumed interests. Google has, for example, worked out that I have a thing about Charles Darwin, so I tend to see a lot of advertisements from creationist nutjobs hoping to convince me of the error of my ways. This is fine by me: I would rather Google have the creationists' money than the creationists have it.
OK, so I fully accept that Charles-Darwin-related advertisements are probably relevant, as far as I'm concerned. But I must admit that I was a little pissed off, to say the least, when, last week, I was presented with an advertisement for Help the Aged stairlifts.
If I wanted nonsense like that, I'd be using HotMail.
Postscript, 2013: Since I wrote this piece, Google have changed their GMail interface. It's now utterly crap.