A technically detailed introduction to Kprobes:
   http://www-users.cs.umn.edu/~boutcher/kprobes/

Many examples of SystemTap (which builds on Kprobes):
   http://sourceware.org/systemtap/examples/

Ubuntu has a problem with Kprobes/SystemTap implementation. 
I had to rebuild my kernel to make it work. Some info here:
   http://posulliv.github.com/2010/02/26/installing-stap.html

NB: You mileage may vary! 

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SystemTap one-liner to print the ESP in a system call:

stap -e 'probe kernel.function("sys_open").call {printf("ESP %x  %s\n", register("esp"), execname() ); }'