ProntoFax

ProntoFax is a pay-per-use fax service for people who need to send one fax, not fifty. Think court filings, IRS forms, medical records releases, and insurance claims. Most online fax services push you into a monthly subscription for something you'll do once a year. ProntoFax charges a flat $2.99 per fax with no subscription and no account.
The part I'm most proud of: you don't need a scanner or a PDF. Snap a photo of your document with your phone and the processing pipeline handles perspective correction, deskew, and cleanup automatically. Payment uses manual capture, so your card is only charged after the fax is confirmed delivered. A failed fax costs you nothing.
The whole experience is available end to end in English and Spanish, which almost nobody in this space bothers to do.