The following are links to sites that are exclusively 1x2x1, or that contain a lot of 1x2x1 content.
Japanese Language Sites:
Gundam Wing art on Pixiv→ Link to the general Gundam Wing tag on Pixiv.net.
Sean-nos Cyberproject → Possibly the most well-known Japanese 1x2x1 art site, featuring the art of Yuuma Kamibata.
English Language Sites:
A Little Piece of Gundam Wing → Dacia's Duo-centric site, featuring lots of 1x2x1 fanfiction.
AboutGundamWing.com → The best online resource for everything related to Gundam Wing. Screencaps, glossary of characters, timeline, merchandise.
It's all there, and more.
AJU Interactive → A Live Journal role playing game which pairs Heero with Duo.
Amy Mizuno's Little Gundam Wing Page → Amy Mizuno's archive featuring 1x2x1 fanfiction and fanart.
Corpus Yaoi → Home of fanfiction by Stargazerlily.
The DHML → The archive of fanfiction by writers from the DHML (Yahoo group).
DSA SITE → DSA's personal site featuring Gundam Wing fanfiction and fanart.
Duo and Heero for Eternity → Terra's site which is in both German and English.
Endless Orbit 999 → The gorgeous artwork of ASIA. (1x2 password=lemon)
FancyFigures.com → Clare's site has a very large section of her 1x2x1 fanfiction.
Flying Away → The 1x2x1 webring.
Gundam Wing Corner → Anime_Gal22's archive of yaoi and gen fanfiction and fanart.
Gundam Wing Diaries → Large archive run by Debs-Dragon.
GW Addiction.com → One of the largest Gundam Wing archives.
GW Fandom.net → Searchable linkage database for yaoi and non-yaoi fanfiction, recommendations, fanart and Gundam Wing sites. Add your own links.
Haven: A Retreat For DuoxHeero Fans → The Gaia guild for fans of 1x2x1.
Hybrid Rainbow → Excellent 1x2x1 fanfiction by sistercacao.
Jaded → fanfiction by Jade.
Kebzero's Notepad → Fanfiction from Kebzero.
Kracken's fanfiction → Art and fanfiction by one of Gundam Wing fandom's best known authors.
Laughing Jackal → Artwork by Duo Radon.
Literate Lunacy → Ralphiere's website featuring a lot of well written 1x2x1 fanfiction.
Prussian and Cobalt → One of Gundam Wing fandom's oldest sites.
Reach for Me → The Heero/Duo fanlisting.
Scathed Light → A small archive of select 1x2x1 fanfiction.
Self Destruction 101 → Dominique's 1x2x1 recommendations.
Shinigami & Wing → jana's amazing 1x2x1 site, featuring fanfiction and more.
Silent Scribblings → Daimeryan Rei's website with lots of long 1x2x1 stories.
Twisted Perspective → Fanfiction by Spooks.
Visula Kei Onnagata → Home of Britt's Gundam Wing fanfiction.
Under the Bridge → Link Worshiper's site for fanfiction and art.
The following are links to mailing lists that are exclusively 1x2x1-centric:
1x2ML → The biggest pairing-specific ML for the entire GW fandom.
1x2ML-Fic → The fiction-only mirror of the 1x2ML.
1x2x1ML → The 1x2x1 Mailing List. (This list is by invitation only.)
2x1domain → The 2x1 Domain.
DHML → The Duo x Heero Mailing List.
HDML → The Heero x Duo Mailing List.
The following are links to Live Journal communities that are exclusively 1x2x1-centric:
1_equals_2_fans → Focusing on the equality of the relationship between Heero and Duo.
1x2_fans → For fans of any combination of 1x2 and 2x1.
2x1_fansite → Specializing in 2x1.
The following are links to doujinshi related sites:
Boxer and Rice → Primarily a Dragonball Z shrine, this site also has one Gundam Wing doujinshi scanned and translated, with more possibly to come in the future.
Yaoi Knights → Features pages and pages of beautiful 1x2x1 doujinshi images.