Leaving your mark (URL) on popular websites is the easiest one - just look for blogs, forums, directories that allow posting.
Paid advertising, if you advertise in the right place, will bring a lot of traffic, but it only keeps coming as long as you keep paying!
Anybody seen
A Free Online Advertising Network | Free Blog Advertising | AdGridwork.com It's an ad exchange - the site owners say they don't know how to make money out of it, so maybe it won't be available forever...