Most of us aren't fans of advertisements. At this point most of us go blank if we even notice an advertisement on a web page, and we hardly ever click them - and no one's about to fall for the "hit the monkey, win an X-box" trick anymore. Sometimes advertisements are necessary, though, and there are a number of ways you can use Flash to incorporate them:
Google Adsense: This popular ad-serving service from Google selects advertisements for you and delivers them to your audience based on complex targeted algorithms. Although it's primarily code-based, it's not hard to incorporate into your Flash site.
Transparent Animations: We may not always be fond of them, but those roll-out animations that overlay on the actual website copy are good for getting attention.
Video Overlay: You can actually create transparent video (no background) that fits into your website and lays over your content, which is not only a neat effect but also a good way to deliver advertising.