I tried many image viewers but FastStone Image Viewer is the best so far. Fast, customizable in totally free. Much better than Irfan IMHO.
http://www.faststone.org/FSViewerDetail.htm

For video files I have been using bsplayer and zoom player in past years but just recently I discovered Gom Player. Amazing piece of software.
http://www.gomplayer.com/main.html

For audio files - powerful files editor for renaming, managing ID3 tags an much more - TagScanner
http://www.xdlab.ru/en/index.htm