Linice
Linice is a source-level kernel debugger for x86 systems with the look and feel of SoftIce for MS Windows. Break into a running kernel, use breakpoints, single step on modules, kernel, or user programs. Outputs to VGA, MDA, X Window, serial terminal.
Please visit my blog at http://www.devic.us/hacks
Linice using VGA text framebuffer
Linice debugger breaking under X-Window