The QEMU Advent Calendars featured a QEMU disk image each day of December until the 24th. Each day a new package became available for download.
Every download contains a little 'run' shell script that starts the QEMU emulator with the recommended parameters for the disk image. Disk images are either contained directly in the download or are downloaded by the 'run' script (you need to have installed 'curl' or 'wget' in that case).
The disk images contain interesting operating systems and software that run under the QEMU emulator. Some of them are well-known or not-so-well-known operating systems, old and new, others are custom demos and neat algorithms.
The 'run' scripts (and disk images if included in the download) were created by volunteers from the QEMU community to showcase cool software that QEMU can run.
To get in touch please send an e-mail to the QEMU devel mailing list at email@example.com or ask in the #qemu channel on OFTC IRC.