As a big focus in 2020 will be on various kinds of elections, I've pulled together voting-related resources from the last three (!) years of thisfinecrew. Many thanks to all the people who originally brought these links and organisations to the attention of the comm.

Targeted support for progressive candidates

SwingLeft (Lets you volunteer to support progressives in Swing Districts, no matter where you live)
Flippable and Local Majority (Both focused on targeting state legislatures where flipping a few seats can turn them blue -- putting Democrats in charge of the next round of redistricting in 2021)
Get Mitch or Die Trying (Donate to the eventual Democratic nominee in 10 seats that could be flipped and one that needs protecting)
Sister District (Matches volunteers in safe blue places with strategic state elections throughout the country)
Emily's List (Helping pro-choice Democratic women around the country run for office and win.)
Get Her Elected (arranges for volunteers to offer their skills pro bono to progressive women candidates running for office at all levels of U.S. government)
Run for Something (Recruiting and supporting young progressives)
314 Action (Encourages scientists to stand for office and helps them win)
Higher Heights (Investing in a long-term strategy to expand and support Black women’s leadership pipeline at all levels)
The Collective PAC (Supporting African American Candidates)

Get Out The Vote, voting suppression and voting reform

Postcards to Voters (A grassroots organization that sends hand-written postcards to potential progressive voters in competitive districts)
Vote Forward (Send letters to traditionally under-represented voters to encourage them to voite)
National Voter Registration Day (not until September 20, but they need money as well as volunteers)
Moms Vote (Engaging women, especially mothers, to vote through activities like postcard campaigns and phonebanking)
Multiply Your Vote (Volunteering opportunities to encourage people to vote)
ACLU (Various activities and campaigns around voting rights)
Fair Fight 2020 (Helping staff, fund, and train robust Democratic voter protection operations in battleground states)
VoterRiders (Helping every eligible voter to get the ID - and confidence - to vote)
866-OUR-VOTE (Works to advance and defend the right to vote and offers on the day support to people having trouble voting. See this post for details of how speakers of various languages can contact them)
Woke Vote (A collaborative of grassroots organizers in the South registering voters and canvassing neighborhoods)
League of Women Voters (Campaign to fight voter suppression)
Voto Latino (Provides culturally relevant programs that engage, educate and empower Latinos to be agents of change)
Color of Change (Working to raise the level of Black community involvement in the next election cycle)
#Knockeverydoor (Helps volunteers start door-to-door canvasses to help build a majority of voters for progressive causes)
Press for adoption of the National Popular Vote proposal

Keep an eye on your voter registration

Some states are very good at dropping eligible voters off their rolls during "normal" updates, so keep an eye on your registration (and remind others to do so).

You can use Vote.org to check your registration or use this awesome post on US Voter Information put together by sathari to find links to a range of election information for your specific state.

Get involved locally

Why you should join your local political party
Find Democraic Events near you

If you know of any other organisations you think are worth supporting, let us know in comments.