I used Expensify or Freshbooks to keep track of receipts, Invoicely to send itemized bills, DocuSign to verify and send contracts on my phone (Soooo many freelance contracts! No printer!), Tape a Call for phone interviews, Rev for transcription, Quip when I wanted a collaborative note-taking interface that was clean, MailChimp to send my newsletter, Shyp for mailings, Google Calendar to keep track of meetings and deadlines, Venmo and PayPal to collect (and make) payments, and Dropbox to organize basically everything else — from photos to reporting notes to my book manuscript, which was written entirely in Google Docs, by the way.