http://www.volunteering.org.uk/ - An excellent resource for a wide range of volunteering opportunities.
http://www.do-it.org.uk/ - Search through over 1,000,000 opportunities to volunteer, and apply online.
http://www.nationaltrust.org.uk/ - A great way to combine a holiday in the outdoors with conserving the UK's heritage and environment.
http://www.btcv.org.uk/ - Organises conservation activities across the UK.
http://www.oxfam.org.uk/ - Takes action in a variety of ways to fight poverty across the world.