A Raspberry Pi  or Arduino is a perfect companion for experiments, but you need some hardware support during development and deployment.

Here I show you the breadboard development tools I have first, followed by deployment boards.

Breadboard experimenters boards

Traditional extenders into a breadboard. From Adafruit and China, 26 pin (yes, that old) and 40 pin with a flatcable to the Pi. I do not like these very much.

Rasp.io Breadboard Pi Bridge – Pi Ports to Breadboard in Numerical Order. Nice setup, can be used with a Pi B or a Pi Zero. Easy to locate the pins with the descriptions on the PCB.
No Dupont wires required (except for +5V). .Blocks the power rows alas.

RasPiO Pro Hat Protect/Position Raspberry Pi Ports. Ideal if you fear damaging your Pi, has current limiting resistors on the GPIO pins.
Disadvantages are that these resistors may interfere e.g. I2C or SPI and the very small breadboard. Seldom used.

A simple and cheap Chinese breadboard solution. Requires Dupont wires and a printed pin layout required. Works fine, affordable Prototype comes close to deployment layout. Recommended.

The Wombat, a great experimenters board. Much I/O on board (serial to USB, ADC MCP3008, 2 button,s 4 output LEDs, power regulator for +3.3V. All jumpered.Well documented single in line I/O connector.
Large breadboard! The best board in my collection! From Gooligum Australia.

Sunfounder China, experiments Raspberry Pi and Arduino Uno or Mega. Solid, large breadboard, recommended.

Arduino Uno experimenters boards, acryl, from China.

Deployment boards

These boards are where the permanent circuit will be soldered on. Essential are the GPIO connector and enough solder islands.

Richard Saville aka AverageMan made a great collection of boards, I have many in use.

A Chinese (40 pin) and Adafruits board (26 pin, yes that old). The Chinese ones are affordable and great for deployment. Recommended!

Mini Mega deployment board. From China.