Ideally, yes. In reality, it's easier said than done.
1. Most unemployed people need a way to pay their bills. This is not possible if you're dicking around with open source projects or volunteer work. Starting a business requires a certain amount of capital. And "making stuff to sell on Etsy" works only for a tiny minority of people who have the creative skills to make cool stuff.
2. When you're unemployed, finding work should become your full-time job. You should get up at 8am and look for jobs, make a list, take a lunch break, then spend the afternoon tailoring your resume for those jobs. Then you can get off at 5pm or whatever and do other things. What most people do however is that they do other things during the day, and then in the evenings they sit in front of their computers with Monster.com open and browse job openings, and maybe send out a few resumes. Which doesn't work of course.
Basically, the only people who can afford to work on something meaningful while unemployed are those who have a significant amount of money saved up. In today's economy, most people who are at risk of unemployment don't.