Dice-rolling simulator
Line by line analysis

Use four spaces to indent python code. Never use tabs.

from random import randint
from sys import exit
import os


It is a good practice to do top-level imports (without importing module members). This way it's easier to understand what package the methods come from.

os.system('clear')
print "Welcome to the dice rolling simulator!"
raw_input("Press enter to begin.")

total = 0

completed = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]


Use list comprehensions.

def roll():


Roll method is confusing. You expect it to roll a die, but it processes iterations of the game. A while loop would be more explicit here.

os.system('clear')
die1 = randint(1, 6)
die2 = randint(1, 6)
global total


Global variables are no-no. Never.

total = die1 + die2
storetotal()
print "Die 1: %d \nDie 2: %d \nTotal: %d." % (die1, die2, total)
print "\n\nRoll again?"
roll_again = raw_input("Press enter to roll again, type 'stats' to view scores, or 'quit' to exit.\n&gt; ")    


You're exceeding recommended 80 column length limit.

if roll_again == "":
    roll()


See how I changed this block in a code below.

elif roll_again == "stats":
    stats()
elif roll_again == "quit":
    exit(0)


Return from the method is sufficient here.

else:
    print "I don't know what that means so you get to roll again."
    raw_input("&gt; ")
    roll()

def stats():
global total
print "2s: %d \n3s: %d \n4s: %d \n5s: %d \n6s: %d \n7s: %d \n8s: %d" % (completed[0],                                                                     completed[1], completed[2], completed[3], 
                                            completed[4], completed[5], completed[6])
print "9s: %d \n10s: %d \n11s: %d \n12s: %d""" % (completed[7], completed[8], 
                                                completed[9], completed[10]) 


Indentation here is completely messed up.

raw_input("")
roll()

def storetotal():


PEP8: store_total.

if total == 2:
    completed[0] += 1
elif total == 3:
    completed[1] += 1
elif total == 4:
    completed[2] += 1
elif total == 5:
    completed[3] += 1
elif total == 6:
    completed[4] += 1
elif total == 7:
    completed[5] += 1
elif total == 8:
    completed[6] += 1
elif total == 9:
    completed[7] += 1
elif total == 10:
    completed[8] += 1
elif total == 11:
    completed[9] += 1
else:
    completed[10] += 1


roll()


Improved code

I simplified your roll method to roll2d6 and moved all the other code into main.

import random
import os


def roll2d6():
    die1 = random.randint(1, 6)
    die2 = random.randint(1, 6)
    return die1 + die2


def display_stats(stats):
    for i, result in enumerate(stats):
        print "%ds: %d" % (i + 2, result)


def update_stats(total, stats):
    stats[total - 2] += 1
    return stats


def main():
    stats = [0 for _ in xrange(10)]

    os.system('clear')
    print "Welcome to the dice rolling simulator!"
    raw_input("Press enter to begin.")
    while True:
        os.system('clear')
        total = roll2d6()
        stats = update_stats(total, stats)
        print "Die 1: %d \nDie 2: %d \nTotal: %d." % (die1, die2, total)
        print "\n\nRoll again?"
        roll_again = raw_input((
                "Press enter to roll again, type 'stats' to view scores, "
                "or 'quit' to exit.\n&gt; "))
        if roll_again.lower() == "stats":
            display_stats(stats)
        elif roll_again.lower() == "quit":
            break
        elif roll_again.lower() != "":
            print "I don't know what that means so you get to roll again."

if __name__ == "__main__":
    main()