Might want to add the .psd format. It is something of a standard now (layering being its primary advantage). Also, TIFF is still used for a lot of things.
It's probably worth adding vector formats to the list. I use vector art quite a bit, but am no expert on the various formats. Breifly, I know the following:
To restate information in the original post: what have been previously described are "raster" formats. This means, basically, that the image is stored (or, at least conceived) as a grid of pixels. The disadvantage is that, well, it's a grid of pixels. Even though the image might look like a particular object, you can't actually manipulate it as one, because it's really just part of a pixel grid. (So, you need to rely on tools and tircks to "extract" just the pixels you want, and so on.) Also, while such images can be made smaller without loss of quality, making them larger results in "jaggies". File sizes also can get very large.
In contrast, "vector" formats describe an image using mathematical paths. This has the advantage that it can be drawn at any resolution (bigger or smaller) without losing quality. Also, the paths can be manipulated as paths, not as pixel grids. So, that thing that looks like a box really is a box that can be independently edited without weird pixel selection tricks. Disadvantages include a more "sharp" look and an inability to do some of the things you can do with pixels (e.g. smudging, some kinds of filtering, etc.) Most vector formats can also embed raster images (i.e. rectangles filled with a particular pixel grid) inside them.
- Scaled Vector Graphics (SVG) - An XML-based standard for vector images. It is supposedly cross platform, but there appear to be a lot of differences between the implementations, as far as I can tell. This may be improving now. SVG can also support time-based modifications (i.e. animation).
- SVGZ - Since SVG files are XML, they compress well, so are often zipped. When an SVG is zipped, the result is a SVGZ file.
- Encapsulated Post Script (EPS) - PostScript (a language used in printers) is a series of drawing instructions. The EPS concept is sort of a way to wrap little snippets of PostScript code into budles that can be used as embeddable images. For a long time this was (and may still be) the standard for distributing vector content. There are/were a number of different "flavors" of EPS for a while, and lots of different EPS interpreters (with widely varying degrees of quality). So, while the format is cross-platform, there are occasional gotchas with moving files.
- Portable Document Format (PDF) - Sort of the evolution of EPS, PDF is much more advanced and built with a different purpose in mind. Meant to represent paginated documents, PDF can also be used as just a graphics format. Like EPS, it consists of a series of PostScript instructions. Though most people don't use PDFs as graphics, they are capable of preserving layers and object characteristics. As an example, a PDF saved by Adobe Illustrator can usually be opened back up in Illustrator, preserving nearly all of the original Illustrator file characteristics (although, this appears to be done by just embedding the Illustrator format as an unrendered blob in the PDF). PDFs have a lot of font-based options, unlike some vector formats, allowing text to render as the creator intended, even if the consumer doesn't have that font installed.
- Adobe Illustrator Artwork (AI) - Since crushing its most serious rival (Freehand), Illustrator has become the king-daddy vector drawing package, and so its file format is often used to distribute vector content (particularly in the clip art world). Originally based on EPS, it has evolved significantly.
- Shockwave Flash (SWF) - A vector format focusing on animation and (these days) interactive elements. It's primary use for cartography is that it is the most reliable way of delivering cross-platform, resolution independent, zoomable maps to the web. Drawbacks include the fact that most people hate it.
- PICT and WMF - Files containing a collection of drawing instructions, using a platform specific drawing system. PICTs are a Mac format, based on QuickDraw. Windows Metafiles (WMF) are a Windows format, based on the Windows GDI. These files are not at all compatible, but many converters exist to change one into the other. These formats used to be fairly common, but I don't see them used much any more.
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