So if I have an envelope with one of those “No postage neccesary if mailed within the US” type marks on it where the stamp should be, can I use if for other mail. For example, could I just put a lable with my friends address on it and mail it to them, or would the post office not really like that to much? Is it even legal???
No.
A postage paid envelope means the owner of the address listed on it pays for the postage. Using a postage paid envelope to send to an address other than the printed address (the company that actually pays the postage) is called mail fraud.
It’s a federal offense. 18 U.S.C. 1341, makes it a Federal crime or offense for anyone to use the United States mails in carrying out a scheme to defraud. You can be fined and/or go to jail for five years. However, if “the violation affects a financial institution, such person shall be fined not more than $1,000,000 or imprisoned not more than 30 years, or both.”
Source: http://www4.law.cornell.edu/uscode/18/pIch63.html
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Also, if you look at some of these postage paid envelopes coming from credit card companies and such, they have tiny bar codes on the reverse side. Those bar codes are not the same as the larger printed bar codes under the main address (used by postal machines to route the mail automatically). Nope, those bar codes on the reverse side, often printed on the flap identidy the person who received the initial mail!
So if you get a junk mail from a credit card company, decide to fill up the postage paid envelope with crap and mail it back to the company (revenge is sweet!), the company knows where the envelope came from! Of course, if you just dump those solicitations in thetrash and someone else does it, you are the one who gets nailed! 
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Are you sure you want to risk 37 cents for five years in jail? With the USPS on terrorism alert, you can bet your chances of getting caught are higher than just a few years ago.
In Australia, and probably also the US, if you want to send one of those letters to its proper address, but you’ve lost the official envelope, you can simply use a regular envelope with “no postage necessary” written on it by hand. That’s fine, but it also brings us to the point that if you want to start sending things this way to other places fraudulently, barcodes aren’t your only concern (this system was around before barcodes). The simple mechanics of the way the system works will prevent you from doing it. Those letters are sent to companies and charities which have a special account organised with the Post Office to enable them to do it. The Post Office tells them how many articles are waiting for them, the company hands over the cash, the PO hands over the mail. So it won’t work for Joe Bloggs of 378 Smith Street.
Also, here in Australia at least, users of this system actually pay a surcharge above the normal postage rate (it costs them nearly double here), which is why you’ll see the envelopes for charities saying “No postage necessary, but your stamp here will be kindly appreciated”.