On guru, there is a sticky in Ventari's Price Check subforum that contains the links to the various rare skin price threads. While some of them are still being used for posts, no updates have been done in quite a while.
This means the "rarity" of an item might be in question. Weapons that were rare are now overfarmed and drastically reduced in price. Your best bet might be to browse the High End sub-forum and see what is selling for over 100k.
I think that there should be a list of rare skins on the wiki sites. Not all weapons in the wiki sites tell if the weapons are rare. Only a few handful actually say if they are. I'm not the 1 to do any editing or starting any of said projects though. Simply no time for it myself. Just a suggestion that may help out.
Voltaic Spear
Obsidian Blade
Emerald Blade
Eternal Blade
Chrystalline Sword
Near Perf States Echovald Shields
Magmus Shields
Silverwing Recurve
Dryad Bow
All tormenteds
I think clockwork scythes
Demon Fang's
I think thats about it nowadays. But again, it depends on what ur definition of rare is. There is then also the rare low req perfect mod weapons in common skins (eg r8 tactics darkwing will sell for over 100k i think), or r7/8/(in some cases 9) 15>50/+5e uninsc's. In the case of r7, and max mod is worth alot.
Aside from the Crystalline Sword, which has been in the game since the beginning, most of the rare weapons seem to have been in GW:EN or some of the latest batch of the 2007 Design-A-Weapon Contest winners.
Luminarus, QFT and FTW.
[ EDIT ]
Forgot the Magma's Shield, and possibly the Eternal Blade (can't remember when that skin was released - have seen it since the beginning on Ghostlies)