Code editor plugin for Firefox and Chrome: ApexLib APEX Developer Addon by Peter Raganitsch
Unfortunately, this doesn't work in later versions of Apex. Too bad, because it's better than the new built-in one...

This site uses Codemirror for syntax highlighting.
Great article by Sven Weller on how to implement this in Apex: Apex 5 – Region with syntax highlighting.

My own Codemirror implementation and upgrade notes can be found here.

Tomeko.net has a collection of online conversion tools:   Base64HEX    Base32HEX    Base32hexHEX    ASCIIHEX    HEXBase64    HEXBase32    HEXBase32hex    HEXASCII    HEXDEC    DECASCII    MD5 calculator    TextCpp    Cpptext    HEXfile    FileHEX    TextHTML ul    CRC8    WAVE generator    Bin decoder

Base64-image.de has an online tool to convert your images to base64:   base64-image.de

Translate APEX has translations of APEX internal messages.
"APEX Internal Messages are messages displayed by APEX engine in APEX applications, such as default errors (e.g. validations, invalid login credentials) or report labels (search button, pagination links, IR action menu etc.). Those messages are listed in APEX documentation (Managing Application Globalization Chapter) - each of them has code and default English value."

This is my favourite tool to build and test regular expressions: regex101.com.

Also nice:

• tomeko.net
• regexplained.co.uk

A great plugin by Philipp Salvisberg for SQL Developer tool for unwrapping PL/SQL code can be found here: https://www.salvis.com/blog/2015/05/17/introducing-plsql-unwrapper-for-sql-developer/.
I used this to inspect some of the internal Apex packages, and it works like a charm for 10g and above. For 9i and earlier I have not found an out-of-the-box solution yet.

The very first time I tried the free service Unwrap it! I got an error, so I gave up on that at once.
In the past I have used Niels Teusink’s Python script unwrap.py and that worked fine. I do prefer a GUI however, so although I am not a fan of SQL Developer, it is more convenient for this type of thing.

I wrote a blog post on Kafka and Kafka Connect for the Itium website, it can be found here (in Dutch): http://www.itium.nl/data-en-analytics/datastromen-de-baas-met-kafka/.

Another blog post for the Itium website, this time on connecting Apex to SAML 2.0 - to can be found here (in Dutch): https://www.itium.nl/uncategorized/toekomstbestendige-apex-saml-2-0-koppeling/.