Free web service CMYK Converter, as its name implies, converts your image files between RGB and CMYK colour models with a simple three-step process.
There are a load of different ways to convert images between CMYK (Cyan, Magenta, Yellow and Key black) and RGB (Red, Green and Blue) colour schemes, but it doesn't get any simpler than CMYK Converter. Just upload your picture, choose your spectrum, and it will spit out a new photo ready for printing.
Designers know how important (and annoying) switching between both CMYK and RGB is, and thankfully programs like InDesign and Photoshop let you handle this task pretty easily — but if you need to print something quickly and it's saved in the wrong format for your printer, waiting for Photoshop to open seems like a century. CMYK converter is just a click away in your web browser, letting you download a version of your picture (in JPEG or TIFF format) in the other colour spectrum with just a few clicks. You probably wouldn't want to use this for serious projects — you'd want to design in the spectrum best for the final medium — but it's really convenient for, say, quick printing on a one-spectrum printer.
Got a good trick of your own for quickly and painlessly converting between the two (we're looking at you, designers). Share it in the comments.