Fraxy

Fraxy is a Freeware arena Shoot 'Em Up that was released in 2006 for Windows PCs, and which centers around user-made content. The game comes with an editor that lets you make your own enemies, then lets you blow them to bits.
It comprises of two modes: Free Play, which pits you against one or more of the available enemies and TRY, which gives the player different scenarios. The latter is not as popular, due to the fact that it's harder to make scenarios than enemies. An add-on that replaces sprites with custom ones exists, meaning you could make effectively anything.