Pixels to Inches Calculator: People are always asking me if an image is big enough for printing. If you can identify the size in pixels, this calculator will tell you its equivalent size in inches for various uses (select 300 dpi under "output devices," that's usually sufficient for printing).
Online Donation Service Providers: A fantastically helpful chart comparing dozens of companies that provide online donation processing. It's not entirely up-to-date, but if you're looking for a vendor, it's a great place to start. Check it out here.
Email Marketing Service Providers: PC Magazine offers another helpful chart comparing ten email marketing services like Constant Contact and MailChimp. If you're in the market for a service provider, check it out here. Here's another one, comparing twenty services.
Language for Photo Releases: Hopefully you're taking lots of pictures to illustrate your work, and hopefully you're getting permission to use those pictures on your website, in your annual report, and other communications. The American Society of Media Photographers offers sample language for photo releases for adults, children, and even pets here.
Glossary of Printing Terms: Just in case you're curious why your printer keeps talking about PMS, you can look up hundreds of printing terms here.
Nonprofit Marketing Guide Articles: Kivi Leroux Miller's Nonprofit Marketing Guide offers a ton of free articles with titles like Trends in Nonprofit Annual Report Formats, Steps for Creating a Nonprofit Email Campaign or E-newsletter, and 10-Point Website Checklist for Nonprofits. They're full of useful information and stuff you knew but it's helpful to be reminded about. Read them here.