Brainworx bx_opto compressor
a great virtual opto
its got the sluggish response you want from an opto compressor with fantastic control including a killer detection circuit.... this is one of those fully featured analog style plugins that can sound like a lot of the hardware pieces in its style. by shooting for one specific type of detection and providing a ton of tweakability it manages to really cover the bases its trying to cover and be a real workhorse. I mostly like this as a channel compressor over a bus compressor. But anyway, check this out on plugin alliance, its great. Gives you the best features of software with the sound of hardware we're all trained to love.