Do not endanger your life, liberty, or financial situation to deliver a flyer. If you come across a newspaper tube on a busy highway, where the only way to leave a flyer on it is to cross heavy traffic, forget it. Don't go door to door onto private property uninvited and unannounced - this should go without saying.
Never, ever put flyers on windshields of cars. Honestly, this is one of my pet peeves when I get back in my car after being parked somewhere, and see a flyer on my windshield wiper. I've actually cussed people out for doing this. People get really funny, or even downright angry, about ANYONE touching their car. I'm one of them. Don't do it. Additionally, if you damage their vehicle while securing a flyer to their windshield wiper, you could theoretically be held liable for the repair in many jurisdictions. Even if not, you should never risk damaging someone else's property to promote your website. I personally know people who have been arrested for attempting this in parking garages in major U.S. cities - no, I'm not one of them. Don't touch someone else's car unless you're their passenger. Plain and simple.
Do not pass out handbills or flyers in other people's places of business without their permission. You probably won't get permission even if you ask, but just don't do it. Most businesses actively prohibit - and have people trespassed for - attempting to solicit on their property, let alone inside their place of business. Even the Boy Scouts and Girl Scouts have to get permission first to set up a table outside.
If you are in the USA, DO NOT put flyers inside active U.S. mailbox unless they have a postage stamp on them. You can actually be charged with a federal crime in the United States for placing anything in someone's mailbox without adequate postage on it. Some ZIP codes are more aggressive about this than others, but seriously, just don't even take the chance. Newspaper tubes, however, are generally fair game, even if they're those ones that are built into mailboxes - just don't put your flyer in the enclosed portion of the mailbox that is reserved for U.S. postal mail. If you're outside the USA, placing flyers in mailboxes may be illegal in your country as well - check your laws.