Best nib holder I've seen:

Tachikawa Comic Pen Nib Holder - Model 36 - White Grip

No surprise it's from Japan, where drawing with ink tools is still so large an industry that nibs and other inking tools are still made with quality. I have several friends who've switched to this and they just love it. One really cool thing is it accepts crowquil/mapping nibs, which have round, circular bodies like this:
as well as regular nibs, which are concave troughs of metal like this:
Most nib holders can only accept one or the other, and the holders for mapping nibs tend to be thin, exacerbating strain during fine work when using a tool used almost exclusively for fine work. The Tachikawa's thick body reduces wrist strain, and the rubber grip makes it easy to hold. Nibs sit securely in it, but the plastic isn't as rigid as on a Speedball holder, so you don't have to jam nibs in or strain to pry them out again. also has a fantastic selection of Japanese cartooning nibs, the best money can buy, unless you go antique hunting. (Fun fact about the two brands of Manga "G" nib: they're literally made across the street from one another. Both factories buy the same steel, mill it on the same machines, and put different brand stamps on them. They're otherwise identical, sort of like Olfa and NT blades, also made in Japan in neighboring factories using practically identical methods. NT's cutters are way better, though.) They also stock the sometimes hard to find Pentel Pocket brush refills at a not-bad-not-amazing price, and sell the brushpen itself at a pretty amazing price.

The Tachikawa is well worth the six bucks, being comfortable and well made, and would be the only nib holder you ever had to buy in your life.