A compose key allows to easily write special characters such as é ž à ō û ø ☺ ¤ ∅ « ♯ ⸘ Ⓚ ㊷ ♪ ♬ using short and often very intuitive key combinations. For instance, ö is obtained using o + ", and ♥ is obtained using < + 3. I wrote WinCompose because I found none of the alternatives (FreeCompose, CKFW, AllChars or Unichars) to be satisfying.