A better question is probably what can't you do? Drupal is easily one of the most extensible content management systems available. Rather than try to make an exhaustive list, let me provide you with a few examples of what I have done with Drupal. I'm sure you can extrapolate from these and come up with your own ideas.

E-Commerce

There are two major e-commerce packages for Drupal that give you a wide range of options for selling products online. Because the e-commerce is tightly integrated with the rest of your website, it is also easy to create good supporting material for your products. You can use Drupal's articles and web pages to showcase uses for your products that people might not have considered.

Newsletters let your customers or prospective customers subscribe to receive news and information about your products via e-mail. Forums help you to build a community of customers, who are often very good at supporting each other and solving problems. Here at Lazarus customers have come up with solutions that allow my Registration Component to be used from Visual Basic, as one example.

Community or Family Sites

I used Drupal to create the Dowling Family Website, which is a way for family members to stay in touch. It has blogs for the members (currently I'm the only one really into blogging there), photo galleries, and a wishlist. The wishlist is really great when it's birthday or Christmas time for the kids. Grandparents, Aunts and Uncles want to get the kids something they want, but they don't necessarily know what the kids are going to want without the wishlist. This has turned out pretty well in reverse too. Now I know what cool tools to buy my dad for his birthday or Christmas. If you saw his workshop, you'd understand that finding something he doesn't have, that he has a use for, is a big challenge.

Brochure Sites

I can't claim any credit for the design, but I had a small hand in getting Murals and More by Sue Schnackenberg up and ready. Sue's site is a very elegant showcase for the work that she's done, and that she can do for her new customers. The Gallery, in particular, is a nice feature.