I use Excel (and occasionally Numbers), but I want to be able to share with users of Google Docs and LibreOffice (and I may want to switch in the future). What's the most compatible format to save my Excel spreadsheets in?

I'm asking as a question here rather than merely Googling for a list of formats that LibreOffice and GoogleDocs support (although I'm doing that too, and will post the answer if no one else does) because there are likely to be hidden "gotchas" that only someone who has experience using all of the above applications is going to know about. Answers that include personal experience will be preferred over those that only post a link to the relevant facts on google.com and libreoffice.com.