iOS5.0+ !!! (eTools Legacy for iOS3.2+, eTools PRO - advanced version with additional info)
eTools is a powerful virtual toolbox which combines a lot of useful reference materials with electronics tools (calculations, conversations, coding/decoding, etc) in one application in your iOS family devices. Professional or hobbyst can easy recive most of necessary informations or calculation results at some simple finger touches.

Calculations
Resistor Color Code.
Chip-resistors (smd) decoding (3 digits, 4 digits, EIA-96).
Axial inductors colors decoding.
Capacitors decoding.
Solenoid Calculator.
Power calculator.
Ohm's Law.
Reactance (XC for capacitance and XL for inductance).
Resistors voltage divider, ideal and with load. Variable resistor emulation. ("E24 rounded" integrated).
Parallel resistors ("E24 rounded" integrated).
Capacitor charging calc.
LC-resonance.
Y-delta conversion
Resistor for LED(s) (with most popular presets).
Operational amplifiers: non-inverted and inverted ("E24 rounded" integrated).
Compute LM317 voltage mode (with variable resistor emulator ("E24 rounded" integrated) & current mode.
Compute 555-timer: astable mode with ratio 50%-100% and astable mode with ratio 0-100%
Wire voltage drop calculator.
Frequency converter.
DAC/ADC converter.
Decibels converter.
Bin-Oct-Dec-Hex converter.
Pinouts
USB 1.x/2.0 & USB 3.0 pinouts and specifications.
FireWire 4, 6, 9-pins.
RS232 (DB9 & DB25 & IDC10).
Parallel (LPT port) / Centronics.
Ethernet (RJ-45) direct & crossover connections.
Parallel ATA (PATA/EIDE interface).
Serial ATA (SATA/eSATA).
AT-keyboard & PS-2 keyboard/mouse.
AT-power.
ATX-power (20/24-pins) & auxilary connectors.
Molex & fan connectors.
SATA power.
SCART.
DVI.
HDMI.
DisplayPort & MiniDisplayPort.
VGA (standart & VESA modes).
S-Video.
RCA (with typical color marking).
Professional Jack (6.3mm) & MiniJack (3.5mm) - 2, 3, 4 poles.
XLR/Cannon (3, 5-pins).
Speakon.
PS-x AV multiout.
xBox360 AV.
iPod 30pin connector
Enhanced mini-USB (EMU)
OBDII
References
SI system.
Symbols & abbreviations.
Electrical conductivity.
AWG & SWG wires.
Maximum current for copper assembly wires.
ASCII codes.
Basic logic gates.
7400 family (while only a list of family with functions and decoding).
78xx/79xx family (basic data, scheme, packages).
Interactive 7-segment display.
Bipolar transistors (nearly 50000 pcs) catalog & select.
FETS (JFET & MOSFET) (nearly 5000 pcs) catalog & select.
IGBT transistors (nearly 1000 pcs) catalog & select.
Ferrite materials.
E(EE)
EI
EQ/ER/ETD
R (torroid)