Some of these are old tools, and some are new, either way I find myself coming back to these resources so maybe you will find them useful too. 1) The Noun Project - Great Icons
Under the Creative Commons License, you can use this wonderful packet of glyph like icons for your mock-ups, prototypes, digital scribbles, whatever have you.
2) http://responsive.is/yourwebsitelinkhere.com - Checking your responsive site design
The web community is beating responsive web design in every line of designer news possible, so you know it's important. This place allows you to see how your site looks on different devices.
Okay yeah, sort of obsessed with icons right now. Deal with it.
4) Typecast - Typography
Pairing fonts is not my strongest point, but this tool makes that task much easier for me.
This site allows you to test out the pattern on there site background. A simple but great UX tool.
Let me know if you have any favorite resources.