xScope
Used by 10 peoplexscopeapp.com ↗

“xScope helps when things don't look quite right and I need a quick measurement.”
Ryan Essmaker Interview, Designer, developer (The Great Discontent) · via Uses This 
“xScope by The Iconfactory and ARTIS Software.”
Jeffrey Zeldman Interview, Web designer, fan of Web Standards · via Uses This 
“I use and love IconFactory's xScope for close inspection of user interfaces I am developing.”
Joe Hewitt Interview, Creator of Firebug, iPhone developer (Facebook) · via Uses This