http://accessible-colors.com/
I've been working a lot on accessibility on a number of projects recently and have found myself using this tool a lot. The great thing about it is that not only can you check a colour, it will also provide the closest alternative should your current chosen colour not reach the minimum requirements.
https://www.screely.com/
Great for creating screenshots for marketing material. This tool will add a nice browser chrome and background to really make your designs pop off the page.
https://www.0to255.com/
Super useful tool that makes it easy to find lighter or darker colours. This is a great tool to save loading up photoshop or sketch just to check the colour palette.
https://www.remove.bg/
This tool is impressive and something I wish I had years ago when I used to spend hours cutting out images using the Photoshop magic wand! As the name suggests, upload and image and remove the background in no time.
https://www.notion.so/
Not really design related but a tool I'm using so much now to manage pretty much everything I do. From note taking, to dos, trip planning, reading lists, watch lists, guitar learning.
By Jonathan Clift, a UX Desginer based in the UK.