I think "rare beers" really depends upon where you are; there's a LOT of beers that are very rare to find available for purchase locally, but TBNs list covers the famous/infamous sought-after ones really well. (Although MANY breweries, especially in the US, are introducing lots of special limited edition beers, and then super limited edition, and super-dooper limited edition beers to introduce artificial scarcity and to be able to keep prices high and to get free marketing, really.
3 Floyd's Dark Lord Stout might be the original beer that is sought-after and rare due to artificial scarcity; I'd also add that I'd consider it the single most over-hyped beers, FULL STOP. Westvlettern 12, especially when aged for a while, IS worth all the hype, in my opinion though. So don't automatically take hyped to = great beer or beer that's worth the price.
Pliny the Elder is a really sought after super high IBU Double/Triple IPA; it's "brother" beer being Pliny the Younger, but as far as I know Pliny the Younger is only available on draft so it's not just hard to get outside the US, it's IMPOSSIBLE as far as I know. Pliny the Elder IS rare but attainable. I honestly find the best way to get a hold of some of the really rare beers is to stock up on some Westvlettern in the Netherlands or Belgium, and then trade it for something else on the trading forums on one of the American websites (BeerAdvocate has a very active trading forum). Some of the European beers are much easier to get here than in the States so use regional supply and demand to your advantage.
Every year I put together 4 "regular" European beers together and trade a guy I know for a Dark Lord Stout (Clotworthy Dobbin, Fraoch Heather Beer from Scotland, something from Brewdog, and one other random Irish Craft beer that he can't get.).
Trades are a great way to go although the expense for shipping and "gray" legality of shipping/exporting beer does add a little risk that your trade might not go the way that you planned...
Westvlettern beers are definitely trading gold for the hard-to-find American beers, though.
Adam