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ActivitySwap is an activity exchange website. It allows you to receive and send "swap offers" whereby you visit their places by swapping your activity with theirs.
Fasting tracker plus a social platform. Great traction on the play store.
Listymail allows users to create mailing lists in seconds. Drawing inspiration from Doodle and other no-account-required sites, ease of use is the main selling point of Listymail. The system works well but is very basic. Only a couple active users.