Open Air Laboratories (OPAL) has some excellent guides to British wildlife.
NatureSpot is a great resource too, with photo guides to most types of wildlife in the UK (focussing on Leicestershire and Rutland). If you can't see the group of wildlife you're after in the lists below, try Nature Spot.
Microscopy UK has a guide to freshwater algae.
The British Marine Life Study Society has a page about seaweeds.
The Fieldguide to British seaweeds is here.
The RSPB website is a great resource for British birders, I link to it a lot in my blog posts. It has a very useful bird guide where you can browse by species or family and a bird identifier to help you identify what you've seen. The bird guide includes sound files, though only for songs, not calls.
You may also be interested in xeno-canto which shares sound-files of birds from around the world.
The British Trust for Ornithology has some excellent bird identification videos on its website.
You can identify feathers from British birds on Featherbase here.
UK Safari has an information page with links to information about UK mammals.
Amphibians and Reptiles
Amphibian and Reptile Conservation has an online guide to British amphibians and an online guide to British reptiles.
First Nature has a guide to British and European fish
The Royal Entomological Society offer a free service to help identify insects.
The UK Pollinator Monitoring Scheme has a useful guide to telling the difference between bees, hoverflies and other groups of insects.
UK Safari profiles some of the British species of beetles.
A Guide to British Soldier Beetles.
Nature Spot has a useful guide to ladybirds here.
UK Butterflies has a page with links to identification guides for all stages of the butterfly life cycle, so whether you have seen an adult, a caterpillar, an egg or a pupa, they can help!
Butterfly Conservation has an A_Z of British Butterflies.
Nature Spot has a good photographic butterfly guide.
UK Moths offers identification guides to 2155 species of moths. Which is a lot.
Wildlife Insite has a useful site to help identification of caterpillars of moths, butterflies and sawflies.
East of Scotland Butterfly Conservation has a Moth ID gallery.
The Nature Spot Guide to Hoverflies.
All About Hoverflies page.
Steven Falk's Flickr album of Hoverfly photos.
The Sawfly website is a work in progress, but does include an (incomplete) species guide.
The Natural History Museum has a neat little guide to bumblebee identification.
Blooms for Bees clear guide to Bumble bees and Cuckoo Bumbles.
The Bees Wasps and Ants Recording Society has a beginners' guide to identifying these groups.
British Nature Guides guide to bees, wasps, parasitic wasps and wasp galls.
British Bugs has an identification guide for true bugs (Hemiptera) of the UK.
Dragonflies and Damselflies
British Dragonfly Society has a guide to Dragonflies and Damselflies of the UK.
UK Safari has a section dedicated to spiders.
The Conchological Society has an online guide to identifying snails and slugs.
There is a website dedicated to photos of earthworks here.
The Woodland Trust has an online guide to British Trees.
The British Wild Flowers website is a useful photographic guide to British wildflowers.
Plantlife is currently building an online guide to British wildflowers and fungi.
Grasses, Sedges, Rushes and Ferns
The British Wild Flowers website also has a useful photographic guide to grasses, sedges, rushes and ferns.
There's also Fern ID to identify British ferns.
Here is a guide to edible fungi in the UK and how to avoid poisonous species.
Mosses and Liverworts
The British Bryological Society offers an on-line resource for identifying mosses and liverworts.
British Lichens is a site to help you identify lichens.
There's a brief overview of foraging for wild food here with links to identifying species that you can forage.
and if you are a forager, you should follow principles sustainable harvesting to make sure there's enough left over for others (people and wildlife!). There are some simple guidelines for sustainable foraging here.
As ever, red text contains hyperlinks which take you to other webpages where you can find out more.