There are many others, but these two stand out:
Verified Voting
Verified Voting is an effective organization that works for legislation and regulation promoting accuracy, transparency and verifiability of elections. The Voting News section and Blog are great. They are “on the ground” in state houses across the country. Visit the site. If you agree they are worthy of support, do what you can to help advance their efforts.
verifiedvoting.org
Fair Vote
Since 1992, Fair Vote has worked for, and won, electoral reforms at the local, state, and national level through strategic research, communications and collaboration. Visit the site. If you aren’t familiar, learn about the benefits of ranked choice voting. Help spread the message and lend them your support.
fairvote.org