Your main resource for Linux/UNIX assembly programming material is:
Do visit it, and get plenty of pointers to assembly projects, tools, tutorials, documentation, guides, etc, concerning different UNIX operating systems and CPUs. Because it evolves quickly, I will no longer duplicate it here.
If you are new to assembly in general, here are few starting pointers:
x86 assembly FAQ (use Google)
CoreWars, a fun way to learn assembly in general
Usenet: comp.lang.asm.x86; alt.lang.asm
If you're are interested in Linux/UNIX assembly programming (or have questions, or are just curious) I especially invite you to join Linux assembly programming mailing list.
This is an open discussion of assembly programming under Linux, *BSD, BeOS, or any other UNIX/POSIX like OS; also it is not limited to x86 assembly (Alpha, Sparc, PPC and other hackers are welcome too!).
Mailing list address is <linux-assembly@vger.kernel.org>.
To subscribe send a messgage to <majordomo@vger.kernel.org>
with the following line in the body of the message:
subscribe linux-assembly
Detailed information and list archives are available at http://asm.sourceforge.net/list.html.