general great stuff...
Project Gutenberg


java..
live testing


html...
code converter
WYSIWYG editor


latex...
latex colors


productivity
greenshot $\rightarrow$ screen shot capture
Coggle $\rightarrow$ Brainstorming
Trello $\rightarrow$ Notes
draw.io $\rightarrow$ easy diagrams


design
meshlab $\rightarrow$ STLs CAD


electrical
4pcb $\rightarrow$ trace calculator
fritzing $\rightarrow$ schematics