I already knew about Tango, but now just found out about Oxygen, which apparently is an "arm" of the Appeal project.

Apparently there are some nice similarities between Tango and Oxygen... both apply this outline to icons, and they seem to use almost the same color palette (Oxygen's palette has a few more colors). A very Firefox-ish thing. One interesting thing in Oxygen is that they have a "normal" color palette, and a "vibrant" color palette. Maybe that's an interesting concept.

Oh well, i just hope these folks shake hands and would create a unified (in one way or another) icon set for the free desktop.