Ok, this is not really a “hack”, it’s just a fun trick to get free wifi.
I was at the Hilton hotel this weekend and the cost for 24-hr of wifi internet was $9.95.
So what’s a web developer to do? Open firebug. 
Look at the screenshot, do you see the 9.95? Yup, some idiot coded the price into the HTML…
Ok, let me change that to 0.95 instead. Submit form.
Did it work? Yup, my receipt shows the internet charge at 0.95 — You can change this to anything you want. You can even give them more money if you wish.
But there you go, free wifi at the Hilton.
I did a similar thing the last time I stayed at a Holiday Inn. Last time, I viewed the source and found the username and password in the source. I used it to craft a post that authorized me for access.
Let me know if you’ve had similar success =)