Brian, there are a lot of hidden gems out there. Obviously no course can be truly 'hidden' if its on the level of PV, ANGC, NGLA etc. but I'm sure everyone here has played some courses that most everyone else here would consider worth their time to play.
But, and here's the rub, being worth their time to play and being worth their time and expense to travel across the country to play are two very different things. Even if its free, its not free if you have to buy a plane ticket, get a hotel room and rent a car.
I can tell you about a great hidden gem called Saddleback Ridge that about 10 miles north from me. It is not on the level of those top named courses but is underrated, quirky, has a lot of architectural interest, plays quite firm and fast in the right conditions, and only gets better when the wind blows hard. If you enjoy the courses discussed in GCA you'd find it is well worth your time to play, and well worth your money since greens fees are only $30 on a weekend. Well, $30, plus a flight to the nearest airport in Cedar Rapids, Iowa, a hotel and rental car. And no guarantees about the weather unless you can leave on a moment's notice (a tip: wait a few months before coming out, there's a couple inches of snow on the ground right now and the low tonight will be around 0
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I could talk all I want about this course - and I actually have mentioned it probably over a dozen times either as an example of something I'm talking about in an architectural thread, or pointing out some of its quirk (6 par 3s and 6 par 5s, including 6 holes in a row that are not par 4s, and only two bunkers on the whole course) when discussions of quirky designs come up. But its typically rather one sided, since while its an interesting hidden gem, its a bit too hidden for other GCA members to come out and play. Maybe if one of the guys who lives in Chicago decides to take a slow drive to Sand Hills along I-80 sometime, but even then I'd have to convince them that this course is worth being a stop instead of The Harvester - I think Saddleback does in some ways exceed The Harvester, but it also has its shortcomings, and I can't claim that overall it is a better course. So it'll forever remain a hidden gem to all but those who live in the area...