Open Source isn't exactly freeware, but those two open source programs do also happen to be free in the monetary sense
Like Mr Mpehisto said, if you have MS Word, you should be able to do it fairly easily in that. OpenOffice.org is a bit of a large download (and unnecessary if you have MS Word) and GIMP doesn't exactly have any templates, so you'd have to make the invitations entirely from scratch.
Besides for that though, I'm not sure about any free or open source programs made specifically for invitations and such. I'm sure they exist though - perhaps you might want to try looking on
www.tucows.com