Running a business is a lot of work. Ideally, you spend as much time as possible focusing on strategic things (and if you are delivering hourly services, billable work). Here's my personal short list of tools to help make running things easier...
Managing money - Xero. Also Personal Capital if you are managing assets, or Mint if you are mainly managing debt. If you are a sole practitioner, use two bank accounts (one for business, one personal), and pay yourself from one account to the other to keep things clear. Then, at the end of the year, dump your data out of Personal Capital or Mint and you'll have everything you need to knock off your taxes quickly with TurboTax. Pay careful attention to how your company is set up to make sure you set this stuff up right.
If you do services and manage billable hours, Harvest. The data will drop into Xero nicely.
Website - SquareSpace. Skip WordPress - too much work to keep it going, and nowadays you can customize everything you could possibly need with SquareSpace.
Small shared databases - Airtable.com. More structure than, say, a Google Spreadsheet or Excel doc, but not as heavyweight as a full app. Very nice clean APIs for devs to access the data if needed.
Office 365 and/or Google G Suite. I use both for pretty different stuff. Dropbox as well, of course.
Only relevant if you are a dev, but a combination of AWS, Google Cloud, and Heroku have worked well. That's easily another post, if not a book. O.o
Generally speaking just about any tool you could possibly want is probably listed on CloudSurfing. As a side note, this is a good one for sanity checking your business idea - for example, you may not want to launch a CRM (81 listed competitors) or Project Management (178 listed competitors) unless you have a very unusual take on the market.