From collectd Wiki

In order to attract developers and other contributors to collectd, I, octo, am giving away free t-shirts to people who have contributed to the project in whatever form.

The deal is simple: Everyone who puts a reasonable amount of work into the project, e. g. by contributing a cool new plugin, will get an immensely cool collectd t-shirt from me, octo (as long as t-shirts last). These t-shirts are not for sale, in order to keep them elitist ;)

How much “a reasonable amount of work” exactly is, is decided by me. You don't necessarily have to write any code to participate in the program – non-coding contributions such as translating the manpages or giving a talk or writing an article about collectd will be considered, too.

Original announcement on collectd's news page.

Contributors

So far, t-shirts have gone to:

• Sebastian Harl
• Richard Shade
• Peter Holik
• Richard W. M. Jones
• Stefan Hacker
• Oleg King
• Michał Mirosław
• Alessandro Iurlano
• M. G. Berberich
• Michael Stapelberg
• Luke Heberling
• Bruno Prémont
• Doug MacEachern
• Fabian Linzberger
• Olivier Jan
• Amit Gupta
• Eric Reed
• Andrés J. Díaz
• Mariusz Gronczewski

Thanks, guys :)