Rogers Mushrooms - http://www.rogersmushrooms.com
Probably the most complete collection of photographs and mushroom information available online.

MushroomExpert.Com - http://www.mushroomexpert.com
Another extensive mushroom identification site. This one focuses specifically on North America. Whilst there is a considerable overlap between UK and US fungi, some of the mushrooms described are not found in the UK. Care should be taken therefore in using the keys on this site to identify UK fungi.

MykoWeb - http://www.mykoweb.com
Another good, but US centric, mushroom site.

BioImages - Virtual Field-Guide (UK) - http://www.bioimages.org.uk/HTML/T156447.HTM
Extensive set of photographs to help confirm identification. However, the site lacks any search facility or identification keys and has be browsed by taxonomy. This makes the site less useful for identifying unknown fungi.

Northern Ireland Fungus Group - http://www.nifg.org.uk
A large collection of species photos but no other identification information.