Two small (texture replacement and/or with small gameplay tweaks) mods for Shadowgrounds are now supported on Desura. In fact, these two are the only released ones - although both installments of Shadowgrounds have an extensive modding support, there's very little modding activity from the community itself, and more SG levels than mods saw daylight.
Shadowgrounds is easily moddable, all you need is to copy appropriate game files to your own directory in "Mods" folder and start changing textures, scripts, etc. There's also one hidden flaw - there are no model exporters/importers released.
On the other hand, source code of Shadowgrounds and its sequel executables are available. Perhaps there's still much room to mod independent games such as SG.
Desura supports mods for other indie games - if you find anything made for indies, you can add it to ModDB/IndieDB and/or drop a line.