Passive ones like the Morley SUCK TONE and can result in ground loops and massive HUMMMMMMM.
The ToneBone and Framptone use active buffers and do it correctly, as does the Voodoo Labs switcher.
If you are good with the soldering iron, you can build your own with a couple of opamp ICs. Using isolation transformers is a nice touch, but I have found that it isn't necessary (so far, but keeping my fingers crossed).
I have a home built one similar to Craig Anderton's circuit in Electronics for Musicians, but lately I have been using a Lehle D.Loop, which allows MIDI control.