There's a wealth of sites on-line where you can find out about how to identify wildlife. So here's a list of some of the useful ones for the UK:
The RSPB website is a great resource for UK 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 entries for individual species include 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 video bird identification videos on its website.
UK Safari has an information page with links to information about UK mammals.
Amphibians and Reptiles
The Herpetological Society has an online guide to British amphibians and an online guide to British reptiles.
Angling Times has a identification guide for British freshwater fish, whether you're an angler or a naturalist... and a guide to British sea fish too!
Insects and Spiders (General)
The Natural History Museum website has a section devoted to information about some of the more common UK insects and spiders.
Pond Conservation has a guide to invertebrates that can be found in garden ponds.
Marine Conservation has a guide to jellyfish.
UK Safari profiles some of the British species of beetles.
Ladybird Survey has a printable guide to British ladybirds and one for ladybird larvae.
The UK Ladybird Survey is a useful reference guide and wants you records of the ladybirds you see!
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!
UK Moths offers identification guides to 2155 species of moths. Which is a lot.
The Natural History Museum has a neat little guide to bumblebee identification.
The Bees Wasps and Ants Recording Society has a beginners' guide to identifying these groups.
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. There is also a British Spiders website, but when I visited it, a message popped up saying it was forbidden....
The British Marine Life Study Society has an online guide to sea living molluscs.
The Conchological Society has an online guide to identifying snails and slugs.
The Natural History Museum has a guide to the 25 species of British earthworms.
The Woodland Trust has an online guide to British Trees.
The British Wild Flowers website is a useful photographic guide to British wildflowers.
Grasses, Sedges, Rushes and Ferns
The British Wild Flowers website also has a useful photographic guide to grasses, sedges, rushes and 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.
The British Marine Life Study Society has a page about seaweeds.
Microscopy UK has a guide to freshwater algae.
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.
Thanks to the Ragged University for, via Twitter, inspiring me to put together this list. If there's anything you feel is missing, let me know...
As ever, red text contains hyperlinks which take you to other webpages where you can find out more.