The Doepfer A-119 is sort of the "industry standard", actually. Typical 1/4" input jack and amplification of external signals to synth level. But the A-119 has a few other tricks...
First up, it's got a comparator which keys off of the incoming signal dynamics. As long as your input signal is over the level you've set on the threshold pot, the module will output a gate. Then there's the envelope follower itself, which translates dynamic levels into a modulation curve so that you can impose the incoming dynamics envelopes onto some other device in the build. Now, where this can come in handy is when you've run out of comparators, or if you just want dynamics without the signal, you can use it for that as well.