Finding the most frequent character in a string

There are many ways to do this shorter. For example, you can use the Counter class (in Python 2.7 or later):

import collections
s = "helloworld"
print(collections.Counter(s).most_common(1)[0])

If you don't have that, you can do the tally manually (2.5 or later has defaultdict):

d = collections.defaultdict(int)
for c in s:
    d[c] += 1
print(sorted(d.items(), key=lambda x: x[1], reverse=True)[0])

Having said that, there's nothing too terribly wrong with your implementation.

If you are using Python 2.7, you can quickly do this by using collections module. collections is a hight performance data structures module. Read more at http://docs.python.org/library/collections.html#counter-objects

>>> from collections import Counter
>>> x = Counter("balloon")
>>> x
Counter({'o': 2, 'a': 1, 'b': 1, 'l': 2, 'n': 1})
>>> x['o']
2

Here is way to find the most common character using a dictionary

message = "hello world"
d = {}
letters = set(message)
for l in letters:
    d[message.count(l)] = l

print d[d.keys()[-1]], d.keys()[-1]