Abiword can convert from the commandline between any file formats it knows.
Convert from Word to plain text:
abiword --to=txt myfile.doc
Make a pdf from a Word file:
abiword --to=pdf myfile.doc
And so on. The results in these cases would be myfile.txt or myfile.pdf. If you want to specify the output name you can do that too:
abiword --to=txt --to-name=output.txt myfile.doc
Convert ODT to Word:
abiword --to=doc myfile.odt
Convert Word to ODT:
abiword --to=odt myfile.doc
In fairness to other answers, it should be noted that AbiWord uses wvWare to handle Word documents, but even the wvWare homepage recommends using AbiWord instead for most conversions.
I hate word processors. This is the main reason I have AbiWord installed.
You might also be interested in unoconv, which is a similar tool supporting formats OpenOffice knows (which would include spreadsheets and the like), but I have no experience with it personally.