Is there a defacto 3rd party utilities library for Apex such as Apache Commons is for Java?
About the closest thing to Apex libraries is the 'Force.com Labs' apps on the AppExchange that are open-source packages.
They're a good source for some specific code. However, they're not typically distributed as library code.
Also, Force.com's Github contains a good set of SDKs and code-source files for linking into SFDC from a variety of languages and platforms.
Apex-lang is about as close to a Java-style library as you can get. Contains several string, database, and collection utilities that mimmick Java functionality. Be aware though, some stuff including Comparing and Sorting collections is out of date with the advent of the Comparable interface in Apex.
In addition to apex-lang, and like you suggest, I typically create or reuse static helper methods throughout my projects. Static helper methods are very convenient for reusing code in Chatter functionality, DML handling, Exception handling, Unit testing, etc.
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