Not my code, but found in an old forum post. Moving to Github so it can be forked, etc.

MultiSpeedI2CScanner.ino

//
//    FILE: MultiSpeedI2CScanner.ino
//  AUTHOR: Rob Tillaart
// VERSION: 0.1.04
// PURPOSE: I2C scanner @different speeds
//    DATE: 2013-11-05
//     URL: http://forum.arduino.cc/index.php?topic=197360
//
// Released to the public domain
//

#include <Wire.h>
#include <Arduino.h>

// scans devices from 50 to 800KHz I2C speeds.
// lower than 50 is not possible
// DS3231 RTC works on 800 KHz. TWBR = 2; (?)
long speed[] = { 
  50, 100, 200, 250, 400, 500, 800 };
const int speeds = sizeof(speed)/sizeof(speed[0]);

// DELAY BETWEEN TESTS
#define RESTORE_LATENCY  5    // for delay between tests of found devices.
bool delayFlag = false;

// MINIMIZE OUTPUT
bool printAll = true;
bool header = true;

// STATE MACHINE
enum states {
  STOP, ONCE, CONT, HELP };
states state = STOP;

uint32_t startScan;
uint32_t stopScan;

void setup() 
{
  Serial.begin(115200);
  Wire.begin();
  displayHelp();
}


void loop() 
{
  switch (getCommand())
  {
  case 's': 
    state = ONCE; 
    break;
  case 'c': 
    state = CONT; 
    break;
  case 'd': 
    delayFlag = !delayFlag;
    Serial.print(F("<delay="));
    Serial.println(delayFlag?F("5>"):F("0>"));
    break;
  case 'e': 
    // eeprom test TODO
    break;
  case 'h': 
    header = !header;
    Serial.print(F("<header="));
    Serial.println(header?F("yes>"):F("no>"));
    break;
  case '?': 
    state = HELP; 
    break;
  case 'p': 
    printAll = !printAll;
    Serial.print(F("<print="));
    Serial.println(printAll?F("all>"):F("found>"));
    break;
  case 'q': 
    state = HELP; 
    break;
  default:
    break;
  }

  switch(state)
  {
  case ONCE: 
    I2Cscan(); 
    state = HELP;
    break;
  case CONT:
    I2Cscan();
    delay(1000);
    break;    
  case HELP:
    displayHelp();
    state = STOP;
    break;
  case STOP:
    break;
  default: // ignore all non commands
    break;
  }
}

char getCommand()
{
  char c = '\0';
  if (Serial.available())
  {
    c = Serial.read();
  }
  return c;
}

void displayHelp()
{
  Serial.println(F("\nArduino I2C Scanner - 0.1.03\n"));
  Serial.println(F("\ts = single scan"));
  Serial.println(F("\tc = continuous scan - 1 second delay"));
  Serial.println(F("\tq = quit continuous scan"));
  Serial.println(F("\td = toggle latency delay between successful tests."));
  Serial.println(F("\tp = toggle printAll - printFound."));
  Serial.println(F("\th = toggle header - noHeader."));
  Serial.println(F("\t? = help - this page"));
  Serial.println();
}


void I2Cscan()
{
  startScan = millis();
  uint8_t count = 0;

  if (header)
  {
    Serial.print(F("TIME\tDEC\tHEX\t"));
    for (uint8_t s = 0; s < speeds; s++)
    {
      Serial.print(F("\t"));
      Serial.print(speed[s]);
    }
    Serial.println(F("\t[KHz]"));
    for (uint8_t s = 0; s < speeds + 5; s++)
    {
      Serial.print(F("--------"));
    }
    Serial.println();
  }

  // TEST
  // 0.1.04: tests only address range 8..120
  // --------------------------------------------
  // Address  R/W Bit Description
  // 0000 000   0 General call address
  // 0000 000   1 START byte
  // 0000 001   X CBUS address
  // 0000 010   X reserved - different bus format
  // 0000 011   X reserved - future purposes
  // 0000 1XX   X High Speed master code
  // 1111 1XX   X reserved - future purposes
  // 1111 0XX   X 10-bit slave addressing
  for (uint8_t address = 8; address < 120; address++)
  {
    bool printLine = printAll;
    bool found[speeds];
    bool fnd = false;

    for (uint8_t s = 0; s < speeds ; s++)
    {
      TWBR = (F_CPU/(speed[s]*1000) - 16)/2;
      Wire.beginTransmission (address);
      found[s] = (Wire.endTransmission () == 0);
      fnd |= found[s];
      // give device 5 millis
      if (fnd && delayFlag) delay(RESTORE_LATENCY);
    }

    if (fnd) count++;
    printLine |= fnd;

    if (printLine)
    {
      Serial.print(millis());
      Serial.print(F("\t"));
      Serial.print(address, DEC);
      Serial.print(F("\t0x"));
      Serial.print(address, HEX);
      Serial.print(F("\t"));

      for (uint8_t s = 0; s < speeds ; s++)
      {
        Serial.print(F("\t"));
        Serial.print(found[s]? F("V"):F("."));
      }
      Serial.println();
    }
  }

  stopScan = millis();
  if (header)
  {
    Serial.println();
    Serial.print(count);
    Serial.print(F(" devices found in "));
    Serial.print(stopScan - startScan);
    Serial.println(F(" milliseconds."));
  }
}