I am reasonably competent at administering Apache, MySQL, PHP, and general Debian administration, but don't know much about kernel issues and know nothing about Xen or IPtables.
I know a bit about ISC DHCP, Samba, NFS, CUPS, OpenLDAP, ISC BIND, PAM, NSS and NIS (in roughly decreasing order).
I am familiar with C, C++, PHP (among other programming languages), shell scripting, Subversion and Cogito.