InterPals - InterPals is a friendly community of over 5 million friends, language learners, travelers and penpals.
BuddyPress - WordPress social networking in a box.
PenPal World - Connect with people all over the world
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swap-bot - Swap-bot is a online service that organizes group swaps and a community of creative individuals.
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.