There is a lot of software, for Mac, DOS, Unix etc. at the Summer
Institute of Linguistics,
http://www.sil.org/computing/sil_computing.html
I can recommend the Concordancer for Macintosh, which is free, simple and
efficient. I don't know what the stuff for PCs is like as I don't use
them much. So far as I know, a lot of the SIL stuff is free, but I
haven't looked at all the details for stuff that I can't use, as you can
imagine.
Hope this helps,
Naomi