What? I don't use Microsoft Excel?
LibreOffice is free as in beer and free as in freedom. You can download it for free here, for Linux, Mac, Windows, or Android. You can also download the source code and improve and compile it yourself. You can donate money, time, documentation, and your own improvements to the LibreOffice community. And yes, LibreOffice can export your spreadsheets to Microsoft Excel ... if you must. LibreOffice can even import Excel ... most of the time, not reliably.
I generally don't export to Excel, because "Excel" is a dozen different and mutually incompatible formats. I spent way too much time trying to wrangle spreadsheets to match a particular version. It is way easier for you to do that wrangling yourself, from a more stable format like LibreOffice.
I like how Bill Gates spends billions to help the developing world. I like supporting the developing world as well, using software that billions of people can get for free. Teaching the whole world how to use free software, and optimize it themselves for their particular needs, is more valuable to them than all of Bill Gates's money. Open source can be an enormously powerful charity multiplier, although Mr. Gates may not see it that way today. Let's help him see a better future. Bill Gates is a very competent person. If he helps the rest of the world become more competent as well, his competence is amplified proportionally; his wealth is able to purchase vastly more competent human attention, and accomplish bigger goals sooner. In my ideal world, we all become as capable as Bill Gates is now, he becomes even more capable, and together we vanquish war, pollution, disease, poverty, and ignorance forever.