CGP Grey
CGP Grey is a youtuber who has made some interesting introductory videos to alternative voting systems.
Nicky Case - To Build A Better Ballot
Nicky Case is a Canadian indie video game developer who has done a lot of work on simulating society. They made an essay and simulator about voting systems.
Ka-Ping Yee - Voting Simulation Visualizations
Ka-Ping Yee is a mathematician who created a way to test reliability in various voting systems by mapping the system out in color on a grid.
ACE Project
The ACE Electoral Knowledge Network aims to increase voting security and reliability in public elections by collecting voting system information.
Fair Vote
Fair Vote advocates for Ranked voting, particularly IRV. Their website has a detailed glossary as well as information about voting systems and voting security.
Election Science
The Center for Election Science advocates for Approval voting. Their website has lots of information about Approval and related jargon.
Range Voting
The Center for Range Voting advocates for Score voting. Their website has extensive studies of voting phenomena, voting system comparisons, and ways to get involved.
STAR Voting
STAR voting is a modified version of Score voting in which the top two candidates in a score vote run again against each other (automatic runoff).
electowiki
electowiki is a website with information about voting terminology and election reform.
Kenneth Arrow's Impossibility Theorem
Kenneth Arrow was a Nobel prize winning economist who famously proved that no voting system is perfect when there are more than three candidates and can give every voter a happy result. That is not to say some systems are better (for some situations) than others. Check the analysis page for info on the most popular alternative systems.