The actual list of output formats supported by your GeoServer instance can be found in the WMS capabilities document linked by the GeoServer admin console home page.

Here is a list of the known formats at the time of writing (provided by GeoServer 1.6.0).

• JPEG - (format=image/jpeg) - example
• GIF - (format=image/gif) - example
• PNG - (image/png)- example
• PNG8 - (image/png8)- example (Same as PNG, but computes an optimal 256 color palette and returns a paletted PNG, that is, the image size is usually smaller)
• Tiff - (image/tiff)- example
• Tiff8- (image/tiff9)- example (Same as TIFF, but computes an optimal 256 color palette and returns a paletted TIFF, that is, the image size is usually smaller)
• GeoTiff - (image/geotiff)- example (Same eal as with TIFF, plus the extra GeoTiff metadata).
• GeoTiff8 - (image/geotiff8)- example
• SVG - (format=image/svg)-
  example
• PDF - (format=application/pdf)- example
• GeoRSS - (format=RSS)- example
• KML - (format=KML)- example
• KMZ - (format=KMZ)- example
• Openlayers - (format=application/openlayers)- example

For GIF and PNG images the actual output is controlled by the palette parameter too, which allows for 256 color images. See more information in the WMS vendor parameter page.

Want to create your own output formats? We have a tutorial just for you.