The canonical example of the category and still one of the most useful. LibreOffice gives you a word processor, spreadsheet, presentation tool, database and drawing app that install locally, work offline and read and write Microsoft formats well enough for everyday exchange. For home use, study and a great many small organisations, the gap between this and a paid Office subscription is smaller than the price difference suggests.
Where it shows is at the edges: complex Excel workbooks with macros and pivot-heavy models do not always survive the round trip, and the interface is functional rather than polished. If your work involves passing intricate spreadsheets back and forth with Microsoft users all day, stay where you are. For everything else, it has been quietly good for years.