Tomoe River paper has an awesome reputation in the fountain pen community. I'd been hearing about it for quite a while, but never really knew where to find it. When I saw that JetPens had a 3-page sample for $1, I couldn't pass up a chance to give it a try. Boy was I shocked when I opened it up and felt this paper...it is about as close to tracing paper as you could imagine. It's a bit "fragile" in that you could very easily crinkle or crease it if you're not careful.
The best thing about his stuff is that it handles fountain pens like a dream. You may as well be writing on Clairefontaine or Rhodia. With that of course comes the extra drying time...but who cares? This stuff is too cool. It's not super practical for daily driver use, but for correspondence you'll definitely impress. There is a Japanese daily planner that uses this paper (Hobonichi Techno) - I may just have to pick one of those up one day if I can commit myself to actually using a daily planner.
JetPens sells the 3-page sample for $1, or a 100-sheet packs for $14.75. They come in both A4 and A5 sizes.