well, if you're a normal person (and since you're on slashdot, you're probably not), if you search for cars once in a while, you'll start getting car ads everywhere you go on the internet.
When I started testing my employer's website, I started seeing a lot of ads for our/their sites which I never saw anywhere before. I'm not allowed to click on them, since that will cost us money.
Maybe the neatest thing was that I just missed my bus last Thursday, and spent a brief sprint chasing it down a few blocks to the next station unsuccessfully. While waiting for the next bus, I started poking around in my Nexus 5 and some random app popped up a full-screen ad for a taxi service asking "in a hurry?".
OTOH, I was browing Amazon for some wheel covers for my aunt's old car a while ago, and the "what other customers looked at after viewing this item" list had a fleshlight. I stopped shopping for wheel covers.
NPR had some article on how Target targets their ad mailers based on purchase history... They can tell by purchase history when people are expecting, and adjust the ads in their junk mail coupon books accordingly. Some teenager's dad got pissed off at them when they suddenly started mailbombing his family with baby products, and then apologized when he found out a few weeks later that his daughter was pregnant... but Target knew first.
Personally, I like what Google's been doing (and not doing) so far with the data they collect... as a nerd/engineer, a lot it just makes sense. But I'm not as concerned about my privacy as most.