A library has to have a comment block at the beginning (nothing before it):

/*!
@title com.your.library.name
@version 0.1
@author Your Name
*/

Then you'll see it in the building blocks.

Lyndel the biggest limit of eval is that you have to update the label that is containing the code. With the libraries you just edit the library .js and it's done. (I frequently forget to use the reload from file button, let alone to update a label with code :). And Crockford says: eval is evil :).