Matrex is a lightweight vectorial spreadsheet: calculates blocks of values, not single cells; it is strongly multithreaded; users can work together on sheets using a server. Adapters to matlab, scilab, octave, R.
XB-exby-eXcelBuilder is a web app to select/format data into tabular or crosstab format, then download to a *.xls file. It is meant to be a lightweight bridge between raw data and a spreadsheet, with simple options for selection/layout of data.