While it's arguable whether or not you need a stylus at all with an iPad, some people prefer one for drawing over using their finger because it offers better hand-eye coordination. To that end, we like the Wacom Bamboo stylus.
You have about a million choices for a stylus for an iPad, and the bulk of them are very similar to each other. What's nice about the Wacom Bamboo is that it hits the sweet spot of not being horribly overpriced and still feeling like it's built well (if build quality doesn't matter to you, then you can always make your own stylus from a pen and some sponge). It's also really comfortable in your hand, and it feels about as natural as a fake pen on a touchscreen could possibly feel.
If you're a fan of any of the popular drawing apps on the iPad, the Wacom Bamboo can make the experience a lot better.