Oh My Startup is a collection of 40 semi-customisable illustrations across a range of scenes (free 8 illustrations).
A large database of free icons (credit required).
A great stock photography resource for photos that don't look so 'stocky'.
A few years ago we found this awesome Pinterest board that's a collection of Photoshop mockups.
Compressor.io is another useful tool for compressing image sizes, with a simple drop and drag interface.
A useful tool for reducing the file size of JPG and PNG images, in turn reducing the load time of your website.
DrawKit is a series of free illustration and icons. The website includes a heap of different styles and situations that are great for websites or social media posts.
Humaaans is a downloadable or online customisable library of human illustrations, where you can customise clothes, positions, colours and more.
Adobe Colour, formerly Kuler, is such a powerful tool for both discovering and creating colour palettes that compliment, contrast or compound.
We use Google Fonts on every website. It's a great resource to find and also download fonts that can be used on websites and for branding.