My first "hacking" success was with an ad-supported ISP called NetZero. The app logged you on to the web and had a persistent ad-banner on the screen. If you opened a full-screen app - in my case Starcraft - it would kick you off the internet.
However, I discovered that if you killed the NetZero application at just the right time (after connecting to the network but before the ad banner was initialized), you could stay online with no ad-banner and pwn some Zerg.