Reaper comes with a plugin called ReaPitch which will do it. I'm pretty sure you can download the plugins as standalone VSTs. I think that in general with these things you will have to trade off some between quality and latency, especially if the notes you want to shift are already fairly low. Considering that a single period of a 100Hz wave is 10ms long, the plugin pretty much has to wait that long to be sure that it's got that frequency. 100Hz is a little below low A on a standard tuned guitar, so to shift all of the lowest notes you need a window even longer.
If you can stand that latency, and don't need to shift too far, then you can get some decent results. Keep in mind, though, that it will be shifting the whole guitar by the same amount, so you could drop from EADGBE to DGCFAD, but not to like open G or whatever. There will almost always be some artifacts, though they might be obscured some by heavy distortion. For quick and nasty it might work "we'll enough". The best solution is, of course, actually retuning the guitar.