Software for writing

I'm wondering if I can conduct some sort of straw poll here.

The question is:
Do you use any special computer programs to help you when writing?

I'm not talking about word processors!

It seems that there is a variety of 'other' software (besides word processors) which some authors use as part of their writing process. For example:

• Fiction Sketch
• yWriter5

So, I'd like to know if (aside from word processors), there is any writing software (commercial or shareware/freeware) which you would consider:

• essential (you can't write without it)
• nice to have (occasionally useful)

MTIA for any contributions you are able to make.

Potential Scribe