The tools listed here are a few that I have found to be quite useful for various teaching and research activities. All of them are free software and are supported on linux, either natively or under wine.
FEMM is a 2D finite element simulation software for electromagnetic field analysis. For its surprisingly small size, this tiny software packs signficant analysis as well as post-processing capabilities. It also supports lua scripting to automate the evaluation of analysis cases. Ideal for students to learn magnetic circuits, electric machine modeling etc.
LTspice is a free simulation software for electronic circuit design, particularly known for simulating analog circuits and mixed-signal designs. It is also a valuable tool in studying high frequency phenomena like power switching device transitions, and the effect of layout parasitics on device switchings.
Vim - the ubiquitous text editor
If you are someone who is not quite fond of using the mouse while editing text, Vim is for you. While it has a steep learning curve, mastering Vim can greatly enhance text editing efficiency, especially when it comes to writing math intensive documents on Latex. You can get most editing tasks done with a few minimal keystrokes. If you are going to do a lot of coding or latex typesetting, it is very unlikely that you would regret mastering Vim. An interesting testimony cum demo for this can be found here.
Gnuplot is a highly customizable, command driven, open-source software for plotting and visualising data. It is especially useful in obtaining simulation waveforms from Simulink or PLECS scopes in high quality for publication. An extensive set of examples that can be used as a reference for various types of plots can be found here