Read numbers in the base of the largest digit and write it using underscores This isn't as much a "challenge" as "a unique problem that there are no built-in functions to solve." Basically, you need to do this:


Read some lines of numbers from stdin. These lines may contain trailing or preceding whitespace, and there may be empty lines. No lines will contain "just whitespace" other than a newline. Each non-empty line will have exactly one number. This:

 123

456\t


(where \t is a tab) is considered valid input.
Read the numbers in the input in base N+1, where N is the largest digit in any of the lines. In this case, the largest digit is 6, so the numbers would be read in base 7, resulting in 6610 and 23710.
Find the sum of the numbers. The sum of 6610 and 23710 is 30310.
Convert the sum back to base N+1 again. We'd convert 30310 to base 7, which would be 6127.
For each digit, print that many underscores, followed by a space. The last digit should instead be followed by a newline or a space and a newline. In our example, that would be ______ _ __\n or ______ _ __ \n, where \n is the newline character.


Here's an example in Python:

import sys

def convert_to_base(n, b):
    'Convert `n` to base `b`'
    res = []
    while n:
        res.append(n % b)
        n /= b
    return res[::-1]

# read the lines, discarding empty ones and stripping any whitespace
lines = map(str.strip, filter(None, list(sys.stdin)))
# find the largest digit, and add 1 to it to make our base
base = max(map(max, (map(int, line) for line in lines)))+1
# read the lines in the base and sum them
result = sum(int(line, base) for line in lines)
# print the result in the original base using underscores, separated by spaces
for number in convert_to_base(result, base):
    print '_'*number,
# print a newline
print


This is code golf: the shortest program wins!
Pyth, 26

JscR).zjd*R\_jsiRKhseSsJJK


Demonstration.

JscR).z
     .z                       Take the input as a list of lines.
  cR)                         Remove all whitespace from each line.
 s                            Combine the lines.
J                             Save the result to J.
                              J is now the list of the string forms of the numbers.
       jd*R\_jsiRKhseSsJJK
                      sJ      Combine J into one string.
                    eS        Find the maximum of that string.
                   s          Convert it to an integer.
                  h           Add one.
                 K            Save to K. K is the base.
               iRK      J     Convert the strings in J to base K.
              s               Add them together.
             j           K    Convert the result back to base K, as a list of ints.
         *R\_                 Map each int in that list to that many underscores.
       jd                     Join on spaces and print.