Coolors are generally my first stop for palette design anyway, but now that I've discovered this quick WCAG contrast checker they've become invaluable. There are a few similar tools out there, but this one has two key features:

1. The ability to alter either colour using an intuitive colour picker, so you can slide through options within a colour space to find where the A/AA/AAA boundaries are;
2. A quick "optimise" button that finds the closest AAA-rated colour combination.

#2 is a bit overzealous, but it does provide a quick way to find some interesting pairings.