These are the companies or individuals contributing a monthly amount to help sustain Neovim development. See the bountysource campaign for more details.
Bountysource does not yet provide a UI for sponsors to inform their URL. Most URLs displayed in the list were obtained using the github API and a heuristic to match the bountysource slug(a sort of sponsor identifier) with the github username, but this method was error prone and failed for many sponsors, including those that don't have a github account.
If you are pledging $5/month or more and want to have your URL listed, please send a PR to neovim.github.io with a new entry for your bountysource slug in the js/sponsors-override.js file. You can obtain your bountysource slug with the following command:
curl --header 'Accept: application/vnd.bountysource+json; version=2' 'https://api.bountysource.com/supporters?team_slug=neovim&per_page=10000&page=1' 2> /dev/null | python -m json.tool | grep -C5 $NAME | grep slugwhere $NAME should be replaced by your bountysource display name(as shown in the sponsor list).
To fulfill the $250/month rewards for the new campaign, they were moved to a separate page.