Where original ideas come from
Move offline things online, such as going from physical bookstores to online bookstores
Start from your own needs; your needs are very likely other people's needs too

The experience economy
If you charge for things and tangible goods, you're in the commodity business. If you charge for the activities you perform, you're in the service business. Only when you charge for the time customers spend with you have you entered the experience business

Two business models for online games
One charges by play time: users pay to spend time with the game and experience it
The other charges for items: users pay to spend time with in-game items and enjoy the gameplay those items bring (the experience business doesn't have to charge by time; an experience provider can charge up front based on the estimated experience time)

Judging whether an industry's barriers to entry are too high or too low
Technology, cost, and control of specific resources. Blogging is a classic example of barriers that are too low

Focus groups
Users' past behavior is more valuable than what they say they want now. For example, a user says they want mutton-flavored toothpaste, but when you actually give it to them, they won't pay for it. Users' past behavior, especially purchases that actually happened, better reflects what they really want

Usability testing
Find representative users who match the profile of the product's potential users. You don't need many testers: when a product's users are very similar, 5 representatives are enough; when the product has many types of users, test 3 users from each group
Ask these representative users to complete some tasks on the product or prototype. For a forum, for example: create a post, follow up on a thread, reply, etc.
Watch them in an observation room: where they succeed, where they get stuck. Don't speak; listen to their feedback

A/B testing
When you want to make changes after launch, use A/B testing: route 10% of visitors into two test environments, A and B. Version A keeps the current design unchanged, and B is the modified design. Then analyze which design better achieves the intended goal; test data helps you find the best option. Example: Google's Website Optimizer.

Three elements of user experience
Don't make me wait
Don't make me think
Don't annoy me