The best replacement for arduino is raspberry pi.

The best alternatives to arduino are rated as following:
★★★★★  raspberry pi
★★★★  pic
★★★  netduino
★★★  basic stamp
★★★  avr
★★★  beaglebone
★★  propeller
★★  picaxe
★★  msp430
★  plc
★  galileo
★  mindstorms
★  parallax
★  mbed
★  beagleboard
★  sainsmart
★  stamp
★  fpga
  ti launchpad
  nerdkit
  funduino
  seeeduino
  redboard
  raspberry
  wiring
  axon
  arduino uno
  basic stamp 2
  gertboard

Your alternative for arduino is not listed? Suggest!
Speaking about arduino, one probably shouldn't forget about ethernet.

Other search terms related to arduino and their ratings are:
★★★★★ ethernet
★★★★★ bootloader
★★★★ wifi
★★★★ bluetooth
★★★★ java
★★★★ labview
★★★★ android
★★★★ lcd
★★★★ raspberry pi
★★★ delay
★★★ matlab
★★★ ftdi
★★ processing
★★ gui
★★ breadboard
★★ xbee
★★ ethernet shield
★★ relay
★ servo
★ arduino
★ visual studio
★ sd card
★ encoder
★ avr studio
★ cmucam
★ touch screen
★ motor shield
relays
led matrix
voltage regulator
sound
stepper motor
ir sensor
resistor
wireless
You have in mind a companion for arduino and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of arduino in all years. Click on graph for more.
arduino
raspberry pi
pic