May be someone is shooting too fast? (not every short question lacks a point, I mean)
I.e., as slim(*) as GhostWall, but w/source code and supported.
-- providing simple port/protocol/direction alllow/disallow rules
What exactly is your problem here? You haven't really stated much and it's fairly vague. It would poll a lot of different opinions and doesn't provide a definitive answer.
You need to be more definite on what you want, and what exactly your problem is. Windows includes what I call a "small firewall" that already does this. A little research and telling us what you have/haven't tried would help.
Just asking for a list of software that will work is off-topic here, as it's basically a shopping question.
If you edit the question, we could vote to reopen it.