Arial with Proxima Nova in action on helloaerial.com

Under the hood

.header-1 { font-family: Arial; font-size: 15px; font-style: normal; font-weight: bold; letter-spacing: 0px; line-height: normal; text-transform: uppercase; color: #F5FF00; }
.header-2 { font-family: ProximaNova-Extrabold; font-size: 93px; font-style: normal; font-weight: bold; letter-spacing: normal; line-height: 102px; text-transform: uppercase; color: #FFFFFF; }

Arial is available from Fonts.com and MyFonts.