OTRS

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License: AGPL3

SIRIOS   (Compare Project)

License: gpl

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Mostly written in PHP

License: devblocks_public_license

Request Tracker   (Compare Project)

Mostly written in Perl

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Work Request Management System   (Compare Project)

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License: gpl

Project HQ   (Compare Project)

Mostly written in Python

License: gpl

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Mostly written in Perl

License: mozilla_public_2_0

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License: gpl

Flyspray   (Compare Project)

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License: lgpl

Roundup Issue Tracker   (Compare Project)

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License: mit