A flavour of the types of apps available for Maemo 4 is available on http://maemo.org/downloads/

There are spreadsheet apps (such as Gnumeric) and word processors (such as Abiword). Maemo 5 (as run on the N900) already has Documents-to-Go available to view Microsoft Office documents. There has been mention that they'll have a chargeable version which allows you to edit, but I've not seen it.