Secure your email communication with free software - A guide for installing, configuring and using Mozilla Thunderbird, Enigmail, and GnuPG to provide secure and encrypted email [GNUs]

Highly recommended. I use enigmail too.

Do you only half-remember a commandline command you recently used? If you use bash (most likely you do), you don't need to look it up again or reinvent it. Just type control-r and then the part you do remember. Bash will search it's history for commands of which what you typed is a substring.

Example: I remember I used some browser to visit some site named (something)fied(something):

ctrl-r fied

and the result is:

(reverse-i-search)`fied': firefox http://www.nuxified.org

Now just press enter to visit everyone's favourite free software help site :-)