TW My sysadmin toolbox
I administer servers and networking equipment in a small to mid-size heterogeneous (but 99.44% *nix) environment. I've worked on projects ranging from NIS-to-LDAP migration for authentication, to the deployment of a 164-CPU Beowulf cluster, to writing an extension to a large OO-PHP application. I'm a generalist, but my favorite areas are LDAP (and authentication in general), database design and administration, and automation (a broad category that includes writing tools and code to glue services together). Here are my favorite tools.
Minicom allows me to use my laptop as a serial console. I can walk up to any server (x86, SPARC, whatever), plug into the serial port on the back, fire up minicom in an xterm window, and log in.
Some readers are going to roast me for using Telnet instead of netcat, but in the cold, cruel real world in which I live, I do not always have a choice as to the installed software. If I'm consulting, I'm not going to tell a client that I need netcat to do what Telnet (which is already installed) can do just fine. If you want to test connectivity to your mail server, you don't have to have a fat mail client (or a mail account for that matter). Just telnet to port 25 of the mail server.