Here you can find a compilation of what that site calls the top 21 free qualitative data analysis software. Some I have heard of, som not. Some can handle both audio and video data.
Any of the typical qualitative data analysis programs can be used for thematic analysis, based on coding relevant data segments and then searching and retrieving based on those codes. Also, all the major programs offer some version of a free trial.
For a truly free program, I would recommend QDA Miner Lite.
As I indicated earlier, any of the typical qualitative analysis programs will perform qualitative analysis. Unfortunately QDA Miner, the best of the free programs, does not run on a Mac.
TAMS Analyzer is a Mac/Linux open source software for qualitative coding :
http://tamsys.sourceforge.net/
Regarding the original question
Nvivo has a 14 day trial period and is both Mac and PC (although the PC version has more features).
on a broader note, if you have access to Parrallels desktop software you can install any PC software on a Mac, for instance QDA Miner lite as was suggested by D L Morgan.
Use of Atlas.ti for large volumes of qualitative data.
It is license-based and bought online. It has subsidized rates for institutions and students. It is very user friendly. I used it for my doctoral research.