Perhaps the easiest way is to use the GDAL tools, which underpin many GIS and mapping systems out there. If you're dealing with lots of files, it is a fairly easy task to write a batch file or script to automate the process.
I'll make some assumptions here about your level of comfort with the command line, but hopefully it should be easy to follow.
First, obtain GDAL. For most Linux distros, there should be a package in your repository. For Windows, it is easiest to download either FWTools or OSGeo4W.
Next, you need to run the program
gdal_translate which will do the actual hard work for you. For your needs, it is a fairly simple set of options:
gdal_translate -of JPEG -co "WORLDFILE=YES" <source_filename>.tif <dest_filename>.jpg
This just tells GDAL to output a JPEG image given your source file and a destination filename. It will create two files, the JPEG image itself, and a .jpw (or possibly .wld) file which contains the georeferencing information, without which it would just be a picture.