Features:

- Quite small form factor (100x32mm or 3.94x1.26in), so its portable and it can be manufactured very cheap (you can panalize the design, so you can make two rulers per PCB)
- Metric Scale (mm) and two Imperial Scales with different subdivisions: 1/8 inches on one side, 1/10 inches on the other.
- Degree scale (6° per line), so you can (roughly) measure and draw angles.
- Different rounding (1, 2, 3mm arcs and 90°) on corners, so you can predict how it looks when you design a PCB's corners.
- Important formulas and unit conversions: Voltage Divider formula, Unit definitons for Resistor, Capacitor and Inductivity. Conversion between inch and cm and mil and mm. Additionally the most important angle values in degrees and radiants.
- Resitor Color Code Table
- A table with different trace widths (1/4 with soldermask, 2/4 without) and its widths in mm and mils. Also the table shows the maximum current capacity of the traces for a temperature increase of 10K and 50K.
- Different drill holes without copper and sizes in mm and mil on the top, on the bottom holes with copper, sizes in AWG and the maximum current for vias of this size.
- Three different pitches (2.54mm, 1.27mm and 2.00mm)
- Different font sizes in mm and mils.
- Important Footprints (KiCAD style): 1206 - 0402, TQFP32, SOD323, SOT23/SOT23-5, SOT223, SMA, SMB, SOIC8, TSSOP16, D-PAK. QFN28, SOT89, SC70, HC49, WS2812 and BGA16.
- 5mm hole for keychain attachment or easy attachment on wall hooks.
- Bottle opener
- Licensed und Creative Commons BY 4.0

PCB rulers are pretty usual novelty give-aways at professional trade shows, I have some on my desk for a decade now :-)