In the Sound On Sound article Sundance posted, they mention using Cranesong's Phoenix tape saturator. I think this is an effect that a lot of people overlook when recording digitally, and why it's easy to end up with overly clean recordings that sound like they're coming out of a computer. A big difference between what we do and how classic albums were recorded is that they used tape, and that's pretty easy to emulate digitally these days.

I'll throw a bit of saturation on pretty much everything, especially vocals, to get a bit of color on the tracks and make them more harmonically interesting. Soundtoys' Decapitator is my favourite and has been a go-to for the last couple years, and I usually have an instance of Waves' MPX Master Tape in the signal chain when mixing and/or mastering as well.

I also invested in a really nice reverb unit a couple years back (Eventide Space), and I'm pretty blown away with how much difference a really good reverb makes, when compared to cheaper plugins or the reverb built into my mixer.

Cheers
Kent
PG Music