Burnout: In today's fast-paced work environment, burn-outs are quite common. So, blame the recent gap on your resume to a burn-out if you had been working non-stop until then. Hiring managers understand that every few years, people in demanding jobs need to take some decent time out to recharge their batteries and that sabbaticals are not offered by all companies. So, they will understand your need for a break.
Personal responsibilities: If you quit your job to spend time with your ailing father or to be home while your kid was growing up, say that. The Family and Medical Leave Act is applicable only to large corporations and even then you can take only 12 weeks of unpaid leave in a year. Hence, it sounds reasonable for you to have quit your job to pay attention to your responsibilities at home. However, now that you are ready to get back to a fulltime job, stress on how you have handled those priorities and why they would not demand more of your time and attention now. After all, the hiring manager needs to be convinced that you will not have to leave the job suddenly soon in order to get back to those responsibilities.
Introspection: Every now and then, we need to sit back and re-evaluate our career and our life. Hence, you could say that you took some time off to re-examine your priorities in life and your passions. You could talk of how by doing so you realized you would be happier specializing in a certain niche aspect of your profession or you would like to switch industries if that's true. And, if you are applying for a job with a not-for-profit organization or an environmentally conscious company, you could say that the time off helped you realize that you want to contribute more to the society through your work.
Finally, if none of the above fits into your situation, I advise you to take up some consulting positions, even if they mean that you work for free. Doing so will ensure that you have some content to fill up the gaps on your resume and that you are current with what's happening in your profession.
