As said above, Valve's Source engine, while a little wonky, can access a ton of assets from Valve's existing games that you can use to make some cool levels. In addition to Source, Unreal also has a fantastic system that you can use, as well as a stocked marketplace where you can purchase assets other designers have made to use in your levels. There are also a few packs on their marketplace that are made by Epic Games themselves that they sell for free! Happy designing!