If you're running 10.6+ you don't need to worry about using Windows on your Mac to at least use Hansoft's .hwp viewer program. Hancom Office Hanword Viewer 1.2 is available as a free download on the Mac App Store, and it's at a newer version than the app which they had for download on their website.
With respect to NeoOffice (and consequently OpenOffice) .hwp compatibility - from what I've read, only .hwp files made with Hangeul Word Processor 97 and earlier can be read and edited in OpenOffice (and NeoOffice, which is derived from OpenOffice). That compatibility broke with HWP 2002 and above...at my school at least, the computers run HWP 2007.
Oh, and just to clarify, NeoOffice isn't totally "free". The developers are asking for a small donation for the latest version (3.2.x), while the earlier version (3.1.x) is still available for a "free as in beer" download. (It is worth supporting them as they've put a crapload of effort into getting the OpenOffice codebase tweaked and optimized for OS X.)