National Citizen Service (NCS) - more details can be found here.
There are also several organisations that offer volunteering opportunities.
VOSCA - lists latest volunteering jobs
Charity Jobs - lists latest volunteering jobs within charaties
Sports Volunteering - volunteering in community sport
Vinspired - Volunteering for 11-25 year olds
You can also contact Volunteering England, Volunteer Scotland, Volunteering Wales, Jersey Charities, Guernsey Charities or Volunteering Matters to find opportunities in your area.