For slideshows, same thing. Some HTML5+JS solutions like reveal.js are really nice. LaTeX+Beamer is good for some stuff.
For spreadsheets, I think Google Docs handles 98% of the use cases of Excel or LibreOffice. The only real complaint I've heard is creating regression lines in plots are much harder than need be. The online storage and collaboration features are hugely valuable. For people doing data analysis type tasks in a spreadsheet, a command line tool like R or one of the R-like tools like python+pandas or Incanter is often a better choice.