This is a work in progress (in particular, the U.S. State Organizations section is far from complete). I've sent this list to every member of the LGBT-and-allies community I can think of. Please feel free to use it (copy, borrow, edit, update, etc.) in order to further the cause of marriage equality. I'll check the comments for new resources to add.
Update: We just posted the Marriage Equality wiki. It's basically this list, in wiki form. All the latest updates are there.
Campaigns & Organizations
Marriage Equality USA
NCLR (National Center for Lesbian Rights)
HRC (Human Rights Campaign)
HRC Action: Million for Marriage
GLAAD (Gay & Lesbian Alliance Against Defamation)
Atticus Circle
Equality Federation
F.A.I.R. (Freedom, Action, Inclusion, Rights)
Freedom to Marry
Marriage Forward
Barbara Jordan/Bayard Rustin Coalition
No on Prop. 8 (original anti-Proposition 8 campaign)
Events
Overturn Prop. 8 (Google calendar)
Keep Fighting Prop. 8 event list
Protest 8 San Francisco
No on Prop. 8 (Southern California)
California protest schedule
Flyers, Posters, Graphics
Majik Jar
Against 8
Join the Impact logo submissions
Social Networks & Online Communities
Keep Fighting Prop. 8 Ning social network
Join the Impact WetPaint community site
Legal Information
U.C. Berkeley Institute on Governmental Studies: Prop. 8
BallotPedia: California Proposition 8 (2008)
Wikipedia: Prop. 8
Petition for Writ of Mandate (and Paul Hogarth's review)
ACLU of Northern California: LGBT issues
Lambda Legal: Protecting Same-Sex Relationships
Boycotts & Protests
Anti-Gay Blacklist (list of companies to boycott)
Mormon Stock Index
SFGate database of Prop. 8 contributors
Mormons Stole Our Rights
Friends & Allies
Mormons for Marriage [Equality]
Affirmation: Gay & Lesbian Mormons
Religious Tolerance
LGBT-welcoming church programs
GayChurch.org
UCC (United Church of Christ) Faithful and Welcoming Churches, Coalition for LGBT Concerns, Open and Affirming church designation
San Francisco Pride: Prop. 8
PFLAG (Parents, Families, and Friends of Lesbians and Gays)
Sisters of Perpetual Indulgence
Blogs
Join the Impact
InterstateQ
Marriage Equality blog
Los Angeles Gay & Lesbian Center: Invalidate 8
Overturn Prop. 8
SFist ("Prop 8"-tagged stories)
Same-Sex Marriage News
Straight, Not Narrow
Equality Party
To Form a More Perfect Union
Huffington Post: Robin Tyler
Photos & Video
Freedom to Marry documentary
Kyte TV "Pride" channel (currently featuring anti-Prop. 8 demonstration videos)
YouTube "Prop 8" videos
Steve Rhodes's anti-Prop. 8 San Francisco demonstration photos
Publications
Advocate: Prop. 8
Bond ("wedding magazine & network for the modern couple")
Gay Weddings
Marriage Stories
365 Gay news & politics: Proposition 8, gay marriage, marriage story submissions
Huffington Post tagged stories:
Facebook
Join the Impact
Courage Campaign: Repeal Prop. 8
Marriage Is a Human Right, Not a Heterosexual Privilege
Repeal Prop. 8: 100,000 Membership Goal
Twitter
NoOnProp8
protest8SF
"Prop 8" tweets
Discussions
Daily Kos: "CA-Prop 8" tag
Daily Kos: "marriage equality" tag
craigslist Queer forum
U.S. State Organizations
Courage Campaign (California progressive politics)
Calitics (Prop 8, Marriage Equality, Gay marriage)
Equality California
Let California Ring
Kentucky Equality (featuring the United We Stand blog)
Equality Mississippi
U.S. States with Marriage Equality
Massachusetts (since 2004)
Connecticut (since 2008)
Countries with Marriage Equality
The Netherlands/Holland (since 2001)
Belgium (since 2003)
Canada (since 2005)
Spain (since 2005)
South Africa (since 2006)
Norway (starting 2009)