Many of these have already been mentioned, but I'd like to mention a few. You'll note a pretty heavy CLAMP bias.
Vapalla's Cels. This is my most frequently visited resources, and it combines great taste, amazing depth, and concise, informative annotations. Vapalla has also been a good personal friend, and her advice has been instrumental in developing my own collecting tastes of late.
Stem Cels, one of the very best Miyuki-chan and CCS collections and rapidly becoming better and better.
Rallihir’s Cel Gallery. Prepare to be amazed, stunned, and awed by some of the finest hankens in American Hands. The Louvre of CLAMP collections.
Drakonia's Cel Lair. One of my early inspirations, again with great depth and lovely choices from all the series she collects.
The Zone, Tex-chan's home gallery (excerpted in her RS Geckoblue gallery). Very impressive stuff, extraordinary annotations, a great resource, especially for Saiyuki, a series that her pan gengas turned me on to.
CHX. Uende has always stunned me with her artistic abilities, both her own, her gallery design, and the cels she's collected. I hope that she can remain with us at least for a while.
Palace of the Sun, Castle of the Moon, Stargarde's private gallery and one of the most loving collections of Yukito/Yue/Kero cels I've ever seen. Another of my early inspirations.
Mentioned many times, but once more:
BlueBlade Anime Art, one of the most diverse and best curated collections anywhere.
Finally,
Baakay's Anime Cels, carefully designed, lovingly curated, often funny, always entertaining.
Let me also mention
The Snake's Den, Snakeskii's wonderful collection which is currently unavailable (I hope because of a wonderful impending update!) Snakey is one of the most generous collectors I've ever met, the first who was willing to collaborate with me on auctions and arrange a personal show'n'drool meeting (with her brother, the medieval swordplay reenactor, who stood guard over her chastity the whole time).
Golly, with so many really fine collections out there, it's a wonder that anyone comes to my gallery. But some people must wander in from time to time: I'm slowly getting closer and closer to 30,000 hits ...
)
) She has quirky taste
its nice to know that the hard work I put into my gallery is appreciated. 
You're so sweet. 
), 
, 
