I like my psionics with multiple interpetations.
Taking my favorite setting, Eberron, as an example, you have first and foremost Dal Quor, the plane of dreams as the most obvious (and least obvious villian) source of psionics. Then, secondary to Dal Quor, we have Xoriat, the plane of madness and all the nummy little illithids running about, trying to end the world.
![Devious :] :]](http://www.enworld.org/forum/images/smilies/devious.png)
If you think psionics, these are the two first sources you can point your fingers at.
But then we have the Couatl, a serious power group manifested primarily as a divine force. What most people regularly forget is that Couatls have 'psionics' in the same vein as Mind Flayers do, and have a conversion section in the XPH. On the other side of the equation, we have the Aboleths, and if you use Keith's
take on Aboleths, you have another psionics tradition to use, diametrical to the Couatls.
And of course, Eberron has Changelings everywhere. Their doppelganger ancestors have shapeshifting and mindreading powers, which translates very smoothly into the Telepath or Egotist traditions. Keith once said (and don't make me look it up, it's in one of his 4 huge Q&A threads on WotC) he was playing a Changeling Telepath who utilized psionics, but has little understanding of what exactly it was, and showed this mechanically with nil ranks in Knowledge (Psionics) or Psicraft.
These are the primary sources of psionics in Eberron, in my mind.