Downloads via descent2.com, other links will point you to the offical homepage, if it's commercial, that is.

Descent 1 Stuff
Descent 1 Shareware
Descent 1 1.0 -> 1.4a
Descent 1 1.4a -> 1.5

Descent 2 Stuff
Descent 2 Shareware
Descent 2 1.0/1.1 -> 1.2 DOS
Descent 2 1.0/1.1 -> 1.2 Win95/NT
Descent 2 1.0/1.1 -> Virge 1.1 (Virge 3D Chipsets only)
Descent 2 1.1/1.2 -> Descent 2 3Dfx (Voodoo Chipsets, Voodoo2 Chipsets)

Descent Tools

DTX 1/2 - Resoures editor (music, sounds, textures) for Windows 3.x and higher.
Descent Manager - The all-purpose editor (HOG management with advanced author informations, DEMo editor, HAM/HXM editor, level reporter, movie extractor, player editor, SaveGame editor and much more). Available are DOS and Win95/NT modules.
Hoggle - HOG manager with basic author informations for Windows 3.x. (needs some dlls, download here)
Yahoma - HOG manager and briefing editor with basic author informations for Windows 95/NT.
Hamm - HOG manager and briefing editor for various Windows versions.
Descent Mission Builder - One of the easiest Descent level builder(Commercial, must buy)
D.E.V.I.L. - A freeware Level Editor (but also confusing)
Polytron - Polygon model editor for Windows 95/NT.
PMView - Viewer for 3D-PolyModels inside HAM, HXM and POL files. (May need these files)
HXMEdit - Robot behavor editor.