For the second consecutive year, Warren Buffett's sister, Doris, is offering a free online course on getting good value for the money you give away to charities.

Her Learning By Giving Foundation and Northwestern University are teaming up to present "Giving With Purpose: How to get the most out of your charitable giving" on EdX's MOOC (Massive Open Online Course) platform. EdX is a nonprofit initiative created by Harvard and MIT.

The class begins on Wednesday and lasts seven weeks. Registration is open to anyone over age 13. Students can expect to spend two to four hours a week if they participate in all the course activities.

Warren Buffett and actor Patrick Dempsey are among the philanthropists who will appear in videos shown as part of the course.

The class is designed to help students "become more purposeful givers and engage more effectively with nonprofit organizations in their communities."

To help them reach that goal, those registered will be able to nominate U.S. nonprofits and then help decide which ones will receive part of the more than $100,000 in grants that Doris Buffett's foundation will award at the end of the course.

In a video for last year's course, Buffett said he's the "wholesaler" and Doris is the "retailer" when it comes to charity.

—By CNBC's Alex Crippen.