Editors

Booster's Lab
by Noxid

An actively updated in-development editor geared towards cross-platform and cross-engine modding.
Booster's Lab v0.3.3.5 (English)
Booster's Lab v0.3.3.0 (Japanese)

Cave Editor
by Wistil

Has a number of features not present in Booster's Lab. Both editors are compatible with each other so it's down to personal preference as to which interface you prefer.
Cave Editor v0.99d
Source Code