Build-it-yourself modular synth package along the lines of Reaktor, Vaz, et al, with two important distinctions: Its Freeware, and features VST-export - you can export your creations (Instruments or Fx) as Vst plugins for use in any VST 2.0 compatible host.
This second point is probably the biggest selling point.
SEdit does not have quite as many modules as Reaktor so far, but they are growing daily, and the ability to redistribute your creations, with custom skins, is great.
Modules range from the usual analog-styled oscillators, vca's and filters through to midi-altering modules, a range of logic-based modules, pre-built effects, and extensive routing possibilities.
A quick check of the links section at synthedit.com should lead you to various people offering their Sedit creations for free.
The SEdit SDK has recently been released, making it possible for any interested party to develop modules for use in SEdit.