As a general rule, I like to write all of the code used by my games/programs myself. Not because I think I am the best programmer in the world, but because – for me – one of the main reasons I program is to learn more about how software functions on the very lowest levels, and if I used extensive 3rd party code (like Unreal Engine for my game, for example) a lot of that stuff that I *want* to learn about would be abstracted away, or worse: still present but obscured and intermingled with the 3rd party code itself.

Coincidentally, this is the same reason I don’t use managed languages like C#. I quite like knowing about things that managed code wants to hide! The increase in development time is not that important to me, I have no external deadlines to meet.

However, there are points beyond which it’s not practical for me to write the code myself, because it’s either A) incredibly complex, B) boring, C) proprietary, or D) so ubiquitous that it really doesn’t need reinventing. e.g.: writing code to decode an MP3 file into raw audio. I could *potentially* do this, but as it belongs to all four categories, I’m really comfortable with not doing it myself!

Currently in my Freepascal development I am using three 3rd Party Code libraries every day. And they are all excellent, both in terms of features and being easy enough that I can ‘plug them in’ to my own lx7 game engine with very little work. Here they are:

DirectX – by Clootie

No, not DirectX itself (that deserves a whole separate subject!), but a port of all the headers/structures/glue code necessary to use DirectX in Freepascal. This gives me low level access to every function and interface used by DirectX itself, and the port is so perfect I was able to build my own graphics engine on top of Direct3D just by studying the (C++) DirectX SDK. Nothing is missing! Everything works! You probably won’t appreciate how rare this is if you normally use C++.
http://www.clootie.ru/

BASSfpc

BASS is an Audio Library that makes it *dead easy* to play sound and music files of many different types. BASSfpc is the FPC port of this. BASS handles MP3s, WAVs, OGGs, and about a million other sound types, and it does it efficiently and with the bare minimum of code needed (literally about 4 lines of code to init the library and start playing an MP3!).
http://www.un4seen.com/

LNet

The newest addition to my Libs folder. LNet provides a set of classes to implement networking, at the very lowest level, without adding any additional features or complexity. That’s exactly what I needed, because I want to add my own features and complexity! There are many many network libraries out there, but (for Freepascal at least) LNet definately gets my vote, for being simple, class-based, and extremely elegant.
http://lnet.wordpress.com/