Join a choir. (There are even choirs for people who can't sing - google 'choirs for people who can't sing' or 'tuneless choirs')
Join a book club. (Try your local library for details of clubs in your area.)
Become a befriender. (Befriending.co.uk for a list of local networks)
Volunteer. (Do-it.org - Volunteering made easy: just type in your postcode and see what comes up)
Join your local U3A. (University of the Third Age)
Enjoy cycling? Meet up with other cyclists on the Let's Ride website.
Join a group: there's something for everyone at Meet Up. (Or start your own...)
Get a dog. (You'll get an instant best friend and you'll meet up with other dog lovers on your daily walks.)