As I noted in a recent blog post, Wizards of the Coast has published enough of the Dungeons & Dragons 5th Edition rules such that you can play a complete D&D campaign without having to purchase rule books. There are some options missing (such as Feats, and a number of the class builds) for which you would need to buy the Player's Handbook, but the free PDFs contain more than enough of the system for a complete game of D&D from level 1 through to level 20. This is great because you can introduce your non-gamer friends to D&D without them having to spend any money.
Even with these free resources, the Player's Handbook is well worth the money and the Dungeon Master's Guide is one of the best from any edition. Obviously, the Monster Manual is over 300 pages of antagonistic wonders... but these will get you started for cheap.
Basic Rules and More
D&D Basic Rules
System Reference Document (SRD5)
Elemental Evil Player's Companion
Unearthed Arcana Mass Combat Rules
Unearthed Arcana article archive:
Hoard of the Dragon Queen adventure supplement
Rise of Tiamat adventure supplement
Princes of the Apocalypse adventure supplement