XML schemas for tag libraries defined in Jelly, stapler, and Hudson are available. Most modern IDEs and other XML editors can use them to provide auto-completion, which improves your productivity.
See http://www.jetbrains.com/idea/webhelp/referencing-dtd-or-schema.html. Note that when you refer to local files, IntelliJ doesn't make copies, so you can update schemas just by overwriting those files. This is convenient for handling schemas that change frequently.
NetBeans 6.7 (starting with the Beta) should offer completion on these schemas automatically.
We have a plugin for IntelliJ IDEA that assists Jelly file authoring.