From C-Command Software:
BBAutoComplete adds word auto-completion to Affrus, BBEdit, Mailsmith, Microsoft Word, Script Debugger, Smile, Tex-Edit Plus, and TextWrangler. You type the start of a word, press a key, and BBAutoComplete types the letters to complete the word. If BBAutoComplete guessed wrong, you can keep pressing the key to cycle through other possible completions. Other auto-completion utilities need to be taught the abbreviations and expansions that you use; BBAutoComplete avoids this hassle by automatically looking for expansions in the program's open documents. This means that it always suggests completions that are relevant to your current task. BBAutoComplete is probably most useful for programmers, who need to remember and type long variable and method names, but it can also help with prose writing. It's useful any time you need to type long words quickly and accurately.
BBEdit 9 has a built-in auto-completion feature, however BBAutoComplete is preferable in some ways. Depending where your cursor is, BBEdit may ignore potential completions, even if the words appear just a few lines away in the same document. BBAutoComplete consistently shows the same completions, and it prefers words from the same document to words from the dictionary. It's also case-sensitive, to narrow the list to the most relevant completions.