The Trinity Rendering Engine is made by Andrew Lucas, which contains a lot more features, which are even included in Valve's powerful "Source" Engine. Trinity Render is now being wisely used in most of the mods.
Its not the first time we see a heavily modified GoldSrc engine version, but it is the first time we see one attempting to actually extend the engine's limitations and flexibilities rather than making it look good.
While Trinity does have it share of graphics improvements, most of the new and modified systems are supposed to increase the engines capability rate, things such has:
The texture loader, that lets you load texture of any color depth, and any resolution, also supporting the .DDS format which is indeed required;
The particle engine, which lets you, through a very extended and complete scripting system, make the best looking particle effects ever;
The prop manager which allows mappers to fill their maps with has many studio entities as they like, since it breaks the engine limit and loads models into the video card;
Shader support, that due to it's extensive usage makes rendering a lot faster, specially for the studio render, other capabilities include dynamic lighting and real-time shadowmapping as well as advanced instruction sets such as SSE;
The watershader, clearly the best I seen, very customizable, and includes reflections, refractions, fog and frustum;
There are a lot of other features, but you better check the documentation for more detail.
All in all Trinity is a very good rendering system, the fastest and the most professional, there are alternatives, it depends what you're aiming for, but Trinity covers a bit of everything and it has lots of potential. Unfortunately, it does have it's minor flaws, even though the engine makes extensive use of shaders, these are not very customizable, the structure and code base of the engine was programmed with a fixed pipeline, and assembly shaders, meaning that the engine does not have much support for new shader code, unless you know ASM which by the looks of it, is pain. This means that while the engine provides better fallback support, it is in fact slower.
9.5/10 A robust and very complete engine.