Sources: W-Williams.com, lizard_little, and various online stores.
Japanese Region 2 DVDs, with original cover artwork by Temari Matsumoto, shown with their reversible (anime-style) artwork covers. I REALIZE THAT MANY OF THESE IMAGES ARE SMALL. But that's why the cross-references below come in so handy!

Image Cross-References:
Volumes 1 through 5: Daikenkyuu artbook scans.*
Volumes 6 through 18: Daikenkyuu 2 artbook scans.*
Volume 20: Animax 2007 Calendar.
Volumes 1 through 20: See the Collector's Cards posted below.
Volumes 09, 15-17, and 25-28: See Animate Drama covers posted below.
Volumes 25 through 29: Newtype Romance Winter 2007.
Some "Season 3" covers: Special Collector's Cards.
Other volumes: May or may not be included among Animate Drama covers (below), DVD Collector's Cards (below), or Region 1 DVD covers (below).
* Both Daikenkyuu artbooks may contain addditional cross-references; be sure to check!

Source: ripped from various online stores, or scanned by me. From left to right:
1. Contains episodes #1-11, plus a special bonus CD and ten postcards.
2. This special send-away box for the second season was only available for a limited time.
3. Another special send-away box for the "second stage first season." I believe this one was given to media journalists in Japan.
4. Box #2. Contains episodes #12-25, plus a special bonus CD and twelve postcards.
5. Ditto.

Special Animate Drama Covers. // Sources: scanned by Sandraian.
These special drama discs were only available on DVDs sold in Animate stores.

Image Cross-References: Newtype Romance Winter 2007, DVD Collector's cards (below), and Region 2 DVD covers (above).

Source: from Animekitty421.
Collector's cards that come with the first press editions of Japanese DVDs.

Image Cross-References: Duh, see the Animate Drama and Region 2 DVD covers posted above! Also, for the image of Yuuri eating a strawberry: Much larger, beautiful scans of this illustration are available from Newtype Romance Summer 2006, and Asuka October 2006.

Source: scanned by lizard_little.
From left to right:
1. Fan Kanshasai: Shinmakoku demo Valentine?! DVD of a special fan event held February 12th, 2006, with some bonus content as well.
2. Kyoh Kara Maoh! Daizenshuu volume 1. The first collected volume of Daizenshuu (recap) episodes, with newly recorded voiceovers by Takahiro Sakurai.

Image Cross-References: V-Day DVD: Official wallpaper. Daizenshuu 1 DVD: Collectible phonecard.

Sources: small scans from various online stores, large scans from Dias.
North American Region 1 DVDs, with artwork used in the original Japanese DVD release.

Image Cross-References: You can find large scans of many of these images in the Daikenkyuu and Daikenkyuu 2 artbooks! Also be sure to check out the Region 2 DVD covers, and the additional cross-references, shown above.