"Begin with the End in mind" 
so said the wise man. Understand your strength and capitalize on it. This is especially true as we all have fairly limited resources in time and money to do everything in this world.

Do you already operate a retail store?
Online eshop may open up additional sales channel for your business.
Do you have a good knowledge in web design?
You may consider setting an online tutorial/tips site.
Are you a big fan and follower of the Maria Sharapova?
You could setup a blog discussing the star recent news and affairs.

The list goes on.

Advertising dollars, affiliate programs commission are some potential revenue generation engines when you site builds up sizeable amount of daily viewership. Again, if you ever wonder why people at engadget (the top 10 blog sites according to feedster) spend so much time and effort updating the blog with gadget news almost every minute, now you know why.

Internet is a great place to share information, and even more satisfying if we are getting paid for the effort. So go ahead and get started, and turn your idea into viable source of income. After all who can say no to extra pocket money.