Fluke Leading maker of handheld DMMS, but also makes fine calibration standards and bench meters.
Kepco Makes solid power supplies that sell for less than HP, Lambda, etc. They have a specialty in analog power supplies such as the BOP bipolar operational supply, which is really a LF power amp.
Krohn-Hite Active filter instruments and other LF phase/frequency stuff.
Keithley Leading make of “sensitive measurement” instruments such as electrometers
General Radio Although no longer in existence, General Radio made many innovative and durable instruments that remain useful today. The link is to my General Radio Fan Page.
Ballantine Labs One of their most popular products is the 323 analog broadband true RMS voltmeter. Not as well known as the HP 3400, it can be found for very little on the used market.