Regardless of what you think about the 37 Signals folks one bit of their advice has always struck me as being particularly insightful and powerful:

Sell your byproducts.

There are two very strong reasons to do this. First, it can be an excellent business. You've created something to solve some problem or remove some pain that your company is feeling, it's extremely unlikely that your company is so unique that it's pain isn't shared by other companies. And it's unlikely your internal tools would be of no interest or utility to other companies.

Second, internal tools are typically of terrible quality. There are various reasons for this but it's a very common pattern due to fundamental pressures and incentives. By selling internal tools you force them to have owners and you force them to have a quality sufficient to be acceptable to the market. This generally vastly increases their quality, which provides a benefit to everyone who uses them, including you.