Source vs Irrlicht

Valve's official documentation website is great for newbies. It demonstrates the pros and cons of the engine (and since the website isn't maintained by Valve, but instead the community, the pros and cons are largely unbiased). There are also a number of pages dedicated to entities used within official Valve games and also community-made mods that were turned into full-fledged games by Valve. These pages explain the ins and outs of how most source programming works. There are also guides for Valve's tools which are both included in Source SDK and in any Valve-developed game.

Since it has precalculated lighting, this engine is great for low end PCs too.

You don't need to code your own door or ladder etc. You can pretty much use every entity used in Half-Life 2 yourself easily.

Of course you can program stuff into source but for level design only, source has a really easy input/output system for your level logic (e.g. doors, trigger when player walks into room...).

Source includes Source Filmmaker, a video capture and editing application.

Many working examples to study and learn. You can easily experiment modifying an example code to grok in full how the engine works. In the beginning of your learning curve, the example code will offer you many useful code snippets.

If you are considering writing your own engine with openGL, you might want to consider Irrlicht instead. It makes many features of any engine worth its salt easy, including events, serialization, nodes, animators, materials, logging, and animation. Bring your own sound and networking.

Especially if you compile it yourself, it can be very light in memory usage.

If you want to start game developing with C++, then Irrlicht is a good candidate since it removes most of the complications in game making in C++. It will let you obtain experience in programming and games at the same time.

This engine supports multiple formats for 3D objects and textures.