If you work with or for a nonprofit organization, then Beth Kanter and her Beth’s Blog are probably already high on your go-to list of resources. Click here for her posts specifically on measurement.
If your nonprofit is just beginning to explore how to use data and measurement to improve your results, then don't miss this post of hers: “How Does Your Nonprofit Use Data to Improve Results?” You'll find an inspirational video (which we’ve embedded here, too) and you may very well recognize your own organization in Beth’s taxonomy of measuring nonprofits.
The Nonprofit Technology Network (NTEN) should also be on your short list of resources. Don't miss this post, which Beth also links to: “Four Ways Nonprofits Can Increase Their Impact with Data,” and Beth includes a list of several more useful articles on the NTEN site in the comments to her post.
Note also that Katie Delahaye Paine and Beth Kanter will co-author a book to be published next year: Measuring Networked Nonprofits: Using Data for Social Change.