There are three modern and usable spreadsheet applications for Linux. OpenOffice.Org, KSpread for KDE users and Gnumeric for the users of GNOME.
If you are looking for a lightweight alternative for OpenOffice.Org you probably should try KSpread and Gnumeric. Both of them are lightweight enough if your hardware can reasonably be used with KDE or GNOME.
In addition, there are some older spreadsheets like Siag and Oleo that might satisfy your needs. I would, however, recommend either Gnumeric and KSpread to be used as a lightweight alternative to OpenOffice if you are using the spreadsheet for any half-way serious calculations.