I've got a password protected Word document, and I've got the password to open it. However, I do not have MS Word or any plans to buy it. How can I easily open and convert this document to something usable on my OSX computer?
The main crux of this problem is supporting a proprietary Microsoft Word feature without using Microsoft Word. It uses in-document encryption, and not any sort of external security routine that may be common to generally protecting files.
The only time I've used something like this recently is with PDF files, but thankfully Preview.app supports document passwords.
Your best bet would be to find somebody that will unlock it via MS Word's password mechanism, and send it to you manually, perhaps using some other more compatible document protection mechanism such as the aforementioned PDF with a read-only password on it, or a password protected ZIP file.
Another hunch I have would be to see if OpenOffice.org supports document passwords to allow you to view them. I can confirm that OpenOffice supports this feature, but this doesn't say it's interoperable with MS Word documents. I can only suggest that you give it a try for yourself. OpenOffice.org is a great alternative Microsoft replacement Office suite with fairly good Microsoft document compatibility.
You will need a copy of Word to unlock it. When password protected Word will encrypt the file, this needs to be unencrypted by Word.
As Jason says you will need to find someone with work to unlock it, depending on what software you are using, be it Pages or what ever it will then need to be saved in a readable format without a password.
You ahve an option to download a trial version and unlock it as well, however this will only work for a certain trial period. If it really is a problem contact the doc producer or buy Word.