Here are some neglected donation matching opportunities I've found:

1. Animal Charity Evaluators is matching all donations to their recommended charity fund this year until their $300,000 budget runs out, and so far only $4k (2%) has been donated

2. Olivia Brode-Roger is matching donations to GiveWell this year up to $3500, and so far only €465 (13%) has been donated 

If you go to Facebook Fundraisers and search for terms like "matching", a bunch of similar donation matching opportunities come up. Most are to charities that seem ineffective, but there might be some other good opportunities out there. I would encourage you to share them if you find any.