https://github.com/nayafia/microgrants is a quite big list for exactly what you need. This was top result in google after a quick search on your exact post title.
See also:
- The Open Tech Fund (US): https://opentech.fund/
- FOSS Host (UK): https://fosshost.org/
- The NL Net Foundation (EU): https://nlnet.nl/
- The Calyx Institute (US): https://calyxinstitute.org/projects/microgrants-and-small-pr...
More:
US government supports one of the biggest open-tech funds which unfortunately has been on pause since last year: https://news.ycombinator.com/item?id=23636498
These are not US-only but worth a look nevertheless:
There is a directory of sorts here: https://github.com/nayafia/microgrants
(Not with deadlines though, as far as I am aware.)