What FOSS text to speech software do you use?

It really depends on your use case. If you want something that sounds pretty okay, and is decently fast, Piper fits the bill. However, this is just a command line TTS system; you'll need to build all the supporting infrastructure if you want it to read audiobooks. https://github.com/rhasspy/piper

An extension for the free and open source NVDA screen reader to use piper lives here: https://github.com/mush42/piper-nvda

If you want something that can run in realtime, though sounds somewhat robotic, you want dectalk. This repo comes with libraries and dlls, as well as several sample applications. Note, however, that the licensing status of this code is...uh...dubious to say the least. Dectalk was abandonware for years, and the developer leaked the sourcecode on a mailing list in the 2000's. However, ownership of the code was recently re-established, and Dectalk is now a commercial product once again. But the new owners haven't come after the repo yet: https://github.com/dectalk/dectalk

If you want a robotic but realtime voice that's fully FOSS with known licensing status, you want espeak-ng: https://github.com/espeak-ng/espeak-ng

If you want a fully fledged software application to read things to you, but don't need a screen reader and don't want to build scripts yourself, you want bookworm: https://github.com/blindpandas/bookworm

Note, however, that you should build bookworm from source. While the author accepts pull requests, because of his circumstances, he's no longer able to build new releases: https://github.com/blindpandas/bookworm/discussions/224

If you are okay with using closed-source freeware, Balabolka is another way to go to get a full text to speech reader: https://www.cross-plus-a.com/balabolka.htm