I've got a system that has been working quite nicely. Basically it's a python server that interfaces with a headless instance of LibreOffice Calc. Spreadsheets are uploaded to a directory that is polled for changes. The python server opens/closes the spreadsheets in LibreOffice. The python client connects to the python server and can modify and read cells in any spreadsheets. Each spreadsheet is 'locked' when it is being accessed so to avoid two simultaneous modifications/requests giving incorrect results.

LibreOffice nowadays works pretty well with excel spreadsheets. It also means that as LibreOffice improves so does my excel sever. I'm also working in the GNU/Linux world so this is a great solution for me.