See some discussion here: https://www.reddit.com/r/dataengineering/comments/o744oi/fivetan_vs_stitch_vs_singer_vs_airbyte_vs_meltano/
Singer taps are the open source connectivity libraries (commercially used by the core developer |Stitch Data, which is an offshoot of Talend).
Singer taps are used by a wide array of projects including: