StereoPhoto Maker is a free portable tool for working with 3D images.
At its simplest, you can use the program to open and view JPS, MPO, STJ and DAS files, as well as anaglyph images in regular formats. It's not the most visually appealing of viewers, but there are keyboard shortcuts to zoom in and out, move backwards and forwards through a group of files, display them as a slideshow, and more.
There's support for viewing 3D images in many different ways: side-by-side, colour and gray anaglyphs, interlaced, above/ below and more. A convenient "flashing" option quickly switches back and forth between the two images, allowing you to get a feel for the 3D effect even if you don't have a viewer.
Your source pictures can be saved in various other 3D formats. Or if you're more interested in 2D viewing, images can be saved as animated GIFs (with the "flashing" effect) or split into their component pictures (left/ right JPEGs, say).
The real power here comes from StereoPhoto Maker's ability to create and adjust 3D images, though. At its simplest, you might open a 3D picture and use the cursor keys to re-align images, one pixel at a time. But if you're working on a larger scale, the author claims StereoPhoto Maker 4.52 can "automatically batch-align hundreds of images and mount them to the 'window'".
You also get the option to create simple web pages which, in theory at least, allow you to share your favourite 3D pictures with others. We say "in theory" as it didn't work well for us, but if you have more luck then this could be very useful.
StereoPhoto Maker isn't always easy to use. Creating new images in particular requires a good knowledge of 3D fundamentals. It works just fine as a 3D viewer, though, and as the program is just a single 3MB executable, it's not exactly going to get in your way. If you might ever need to work with 3D images then we'd recommend keeping a copy around.