Um, most places...try reading around the forum a little. Off the top of my head---scribd, squidoo, hub pages, quizilla...just think about it a little bit.
Most forums don't allow affiliate links in posts. Some allow in sigs.
As the other OP said, you can post links on some social networking sites.
However, just running around posting affiliate links is not a good way to generate revenue. In fact with some affiliate programs including Amazon they won't even pay you for links you drop elsewhere and will only pay you for links placed on your own sites.
So the best strategy is to build your own site and market that. Don't just drop affiliate links.