Photo Data Explorer is a simple portable photo viewer which also displays just about all the EXIF metadata it contains.
As the program is mostly about viewing photos, its file format support is rather limited. It can't view PNGs, for instance, and after JPG, TIF and BMP you're down to formats such as ICO, ANI and PCX.
Otherwise, though, Photo Data Explorer works much like any other viewer. You can open individual images or photos; zoom in, zoom out, view them actual size or fit an image to the program window; rotate or flip your photo. You're able to step through a folder of photos to view them all. And most of the key options have convenient keyboard shortcuts, so for instance pressing Home displays the first photo in a folder, PageUp and PageDown step back and forwards, and End takes you to the last in a set.
Photo Data Explorer's main selling point is that it pays great attention to EXIF data. The Summary tab displays the main tags (Title, Author, ISO, Flash, White Balance and so on); if you need something else then the main EXIF Properties panel displays 50 tags; and you can manually add an EXIF comment by clicking the Comment toolbar button.
And as a bonus feature, you're also able to resave the current photo. There is support for saving it as a PNG image here, as well as a Photoshop PSD file and a range of more unusual formats (TGA, PIC, CEL, SGI and more).
We found a few interface oddities here, perhaps because the program is listed as a beta, but otherwise it seems a straightforward photo viewer. If you need to see the EXIF data in an image and your current viewer doesn't make this easy then Photo Data Explorer might be worth a try.