It can do warm and fuzzy all the way to damn nasty.
Voxengo’s voyage into tube simulation uses a topology based on the type of asymmetrical tube overdrive normally found in single-tube mic preamps. Download page (Windows 64/32-bit VST | Mac VST, AU).It’s just as useful for tracking as it is for mixing. The Baxandall EQ section can be pre or post the preamp circuit, and it’s capable of up to 4x oversampling. This means you can drive it just like you would a preamp. The SGA1566 is built to respond just like real hardware. Impart the timeless tone of a tube mic preamp to your mono or stereo digital tracks. There is also a limiter for peak control, a sidechain EQ up to 250Hz, and two voicings – Modern and Classic. It’s a workhorse for sure, with two main controls over Dynamics (compression), and Saturation. It can also emulate the vibe of overdriving a single-tube mic preamp. It is not (at least in name) a direct “tape simulation” plug-in, but it does model all of the elements of plug-ins that are trying to nail the tape sound like added harmonics, saturation, and compression. Ferric TDS might be the most fully featured freeware saturation plug-in on the market.
As far as the best free saturation and tape simulation plug-ins go, Variety of Sound has a few options.