Limiting your designs to a few number of colours makes it look a lot more professional, clean and organised. The trouble I have is finding colours which mix well together.
PLTTS is a simple web app which helps you find great colours for your design projects. You can look through the most common palettes or search for your own. You can put in your own base colour in to the form on the top right and it'll give you colour palettes which use that colour.
There really isn't much else to it but it can be a very valuable tool to designers.