Here are some ideas for businesses you can start for less than $100. Most only require a website and your time to get started.
Consulting/coaching - if you have some expertise to share, chances are you could find some customers who are willing to pay for that expertise. A website will help you find clients and describe what you do, but that's not even a requirement to start consulting.
Affiliate sales - check out clickbank or Commission Junction and you'll find that there are 1000s of companies out there who will compensate you for selling their products. All you need to do is create a website and add some value (original content, reviews, etc.). Affiliate marketing is literally a $13 billion dollar business in 2009.
Independent software vendor (ISV) - ISV is the name for very small software shops. If you know how to write software, there's no reason you can't create a product and sell it to customers for virtually no startup costs these days. Sure, a budget might help you hire people and pay for advertising, but plenty of ISVs are building great applications for next to nothing.
Blogging - you can get a blog up for free on hosted platforms, or for way less than $100 on your own domain name. Blogging isn't an easy way to make an income, but can be a great platform for other things, or to sell your own books or information products.