Angel Investors.

Via Gary Will's post at WatStart, an investor handbook (PDF), put out by the National Angel Organization.

For entrepreneurs considering dealing with professional angels, this is a good resource covering the types of issues and considerations that you'll be involved with if you go down that road. Most early stage entrepreneurs these days would probably come away from it with a better appreciation of the value of bootstrapping! As Gary notes, the tone reflects a certain inflated self-importance that is certainly not universal.