Easy to break if the underlying language and alphabet is known

Alphabet can consist of grouped letters

Caesar ciphers can be cracked immediately in the command line using 'caesar' from the bsd-games package

  for i in {0..25}; do echo "Ackkmaa" |  /usr/games/bin/caesar $i ; done 

Substitution can be cracked using Cryptool using letter frequency, digram/trigram frequency and word frequency but a large portion of text must be given as input

Vigenere can also be cracked using Cryptool (for English and German texts)

Strings/files xored with a random key cannot be cracked if length(key) == length(text)

If length(key) << length(string) xortool can be used to infer key length and possible keys given the most common byte: (0x20 for text and 0x00 for binary)

Autocorrelation can be used in Cryptool as well