My fav is the UNITS program from staticfree.info. It looks like a simple calculator (all you see on the screen is a keypad and display window) but there's the UNITS button, so that you can add units to the numbers and the app will convert among hundreds of engineering and physical units. It even knows physical and engineering and astronomical constants so you can do things like solarluminosity/c2 to calculate the mass Sun loses to radiate as energy
with everyone using andriod apps, its high time that some special apps are made for engineering. It will be nice to capture the signal waveform of say clk or some other critical signal during debugging or board bring up and send to your boss for review if he is travelling and he reviews on his smartphone and gives comments.
Are there any apps that enables viewing the PCB layout design files (gerber files) on mobile phone to tab? I understand that there is a constrain on the size of the files as these are big in size usually, but now a days we are getting phones & tabs with sufficiently larger memory as compared to even the PCs dated 5-10 years back.