The Irrlicht Engine is an open source high performance realtime 3D engine written and usable in C++. It is completely cross-platform, using D3D, OpenGL and its own software renderers.
There are definite porting problems to Linux: This library will not be able to use system libraries (-lpng, -lz) unless you heavily modify the build system. There is currently no configuration system in place (like autotools, Scons or Cmake), which makes it a nightmare to compile and the library name is non-standard. Seems to have been made specifically for Windows, with *NIX not considered.
You have to be careful with Irrlicht. It will draw you in with its extraordinarily easy to use interface, and you'll be tempted to write a large scale project with it. I did this myself, and then about half way into writing my game, discovered that the Irrlicht engine is indeed too slow for what I was attempting to make (with 30% of the run time being wasted in the animation system, as of version 1.5). If you're going to use this engine, make sure you stress test it first to make sure it will scale for your purposes, and plan on modifying the engine as soon as you start running into problems.