Andrew Chen on a few interesting areas for online advertising companies: 7 ideas for billion dollar companies in the online ads. "Here it is, not in any particular order:
1. Online video
2. User-generated content (blogs, social networks, etc.)
3. Monetizing international traffic
4. Vertical ad networks
5. Cross-channel/cross-property targeting
6. Continued securitization and efficient brokering of ad inventory
7. New platforms and categories"
Read more on the ideas at Futuristic Play.