One of my sites is placed first on the first page in Google. I have noticed that a company has "hijacked" my domain name, as a title for their "sponsored links", even though this company has absolutely nothing to do with what my site is about and my domain name. Is that legal according to Google?

Depends on if your name is trademarked or not

When I get time I will contact Google and return on this thread. According to what I have read this is clearly against their adword rules.

No, it happens all the time and they didn't Hi-jack it if it's keywords like "Make Money Online"

Anytime you start a business and you are serious about it trademark the name, otherwise you will have problems in the end, so if you do not have a trademark then they did not hijack anything.