I like gnome-terminal and guake, although gnome-terminal seems to be a little "heavy" on resources but I run it on an old T20 and it's running fine. Guake is similar to Yakuake and Tilda that, as previously mentioned by "The Journeyman geek", is a drop-down terminal.
Gnome-terminal, as well as Konsole, has a lot of possible configurations from the GUI but, in the case I was really out of RAM and/or CPU, I would use a simple tty, invoked with ALT+CTRL+F1 (and back to X with ALT+CTRL+F7).
What you will choose? :D
Regards
EDIT: your question arose one of my ancient eye-candy need: a terminal as a desktop background! and I found this: hope you like it!