Das Werkstatt

Since LibreOffice Writer is intended for writing, finding the formatting options for table cells is a bit different then in LibreOffice Spreadsheet.
Here's a short and simple HowTo for setting number format in table cells within an OpenDocument Text (.odt):

1. Select the table cell(s) you want to format.
2. Open the main menu: "Table > Number Format"
3. Select the desired format (should look familiar from the Spreadsheet application)
4. Select "OK".

I'm writing this down, because I always expect this to be in a context menu (right-click) of a cell - but it ain't.