CTRL+R in BASH for searching/activating previously executed commands (the contents of ~/.bash_history).

This is often extremely helpful. Running this alias will serve the PWD up over HTTP (indexed) on port 8000:

alias webserver="python -m SimpleHTTPServer" 

And because I run make all the time, and spaz out and type too quickly, these aliases are probably my most used (seriously):

alias maek=make alias mkae=make alias meak=make alias amka=make alias akme=make 

And probably my most frequently used piece of BASH is a simple script I call upload. I use it to blit any kind of content to my Linode, and it copies the resulting HTTP URL to my clipboard (middle click). Very useful for pasting stuff to people in IRC:

scp -r $* $user@$host:public_html && { URL="http://$host/~$user/$(basename $1)" echo "$URL" xselection -replace PRIMARY "$URL" } 

Just a couple. I can post much more later, must get back to work!