The U73b was a famous German broadcast compressor that was used for mastering purposes during the '60s to the '80s. It featured an all-tube design with adjustable release time.
Now it's back as a plugin courtesy of Audified. Promising the same 'circuit' as the original, this emulation adds input/output gain controls, a VU meter and a sidechain option.
Although the U73b has a simple interface and looks pretty easy to use, it's said to offer an immediately audible sound and can be used not only for mastering, but also when you're mixing drums, bass, guitar and vocals.
Find out more on the Audified website, where U73b can be purchased for $149, and a demo downloaded. It's available in VST, AU and AAX formats for PC and Mac.