Watabou Mansion generator - it's not complete, sometimes it fails to work properly, but this generator aims at generation of a 3d model of a house based on a blueprint. (watabou.itch.io)
Inkarnate - mapmaker. In capable hands, it's fire. For the rest of us, it's like map editor for Heroes of Might & Magic III. (inkarnate.com)
Ambient Mixer - massive (thousands) of free ambient soundscapes to mix and use in your sessions (www.ambient-mixer.com)
Homestyler - at first this floorplanner might look like an odd choice for RPG aid. But then you recall that plenty of games/settings take place in modern reality, or near future. (www.homestyler.com)
Mini Roleplay Adventure - a gallery of 8-bit like classic RPG character classes and creatures (imgur.com)
Incompetech Graphpaper - ever needed to print some special grid? You will, probably, find it in this library (incompetech.com)
D&D Loading Screens - a collection of funny notes for players to consider while the story progresses to the next scene (imgur.com)
Horrorli - small collection of highly atmospheric ambient loops that are perfect for a horror session. (horrorli.fps.hu)
Roll 1d12 - pretty much every entry on this blog is a 1d12 random table. They usually provide quite interesting and original entries. (roll1d12.blogspot.com)
Heromachine - an editor that allows to build a character avatar. Since FLASH technology is dead, HM is now offered as a free download that can be run offline. (www.heromachine.com)
Hanedo's Portrait collection - it's hosted on PINTEREST, but the library is massive and substantial enough to warrant visiting it at least once. (www.pinterest.com)
Dyson's Dodecahedron - a library of a few very well made maps by none other than DYSON (dysonlogos.blog)
David Rumsey Map collection - huge library of old maps that might be used directly, or as a source of inspiration. (www.davidrumsey.com)
Stone Sword - yet another dungeon and world mapmaker that uses tiles instead of objects (www.stonesword.com)
Floor Planner - a mapmaker that comes with the library of modern objects. Good for contemporary settings. (smallblueprinter.com)