I think the general answer to that question is: modules which solve problems in a domain which is

• General. This means a technical domain which is usable across many business domains.

• Complicated and big. This means e.g. implementations of protocols and languages (programming and markup).

By learning to use these modules you both learn a bit of the domain, and you don't get to make the (sometimes fatal) mistakes on your own.

Domains matching the points above are: databases, network interaction (Internet/Web), date/time, Unicode, XML.

/J