I cannot provide the name of a posek allowing the use of pirated software. Most Internet references to rabbanim allowing this are anonymous and along the lines of "a friend has told me", "I heard from", etc.
@thepoosh cites R Dov Lior as allowing it but it is not at all clear that the laws of asheivat aveida (lost objects) apply to this case as the publisher didn't lose the software; instead it was stolen from him. The analogy with iyush (abandon) may not apply either as the publisher would very likely ask the site to take down stolen software if he was aware of them. R Dov Lior has a history of controversial statements (cf. his Wikipedia entry) and without taking away from his greatness in Torah he is not a mainstream posek.
Instead, and for the avoidance of doubt, there are many poskim who explicitly forbid downloading copyrighted software, music or video.
For instance R Moshe Feinstein (Igrot Moshe, OC vol 4 40:19), prohibits copying physical items of music, movies, art, etc. to prevent others from losing money (see comments below for possible limitations to the analogy with online media).
The Rabbinical Council of America came out against this very clearly stating
Whereas the Internet has also enabled people to illegally download
intellectual property covered by copyright, particularly music,
thereby depriving record companies and artists of royalties due them
by law; and whereas such downloading and deprivation of royalties constitutes
theft which is clearly prohibited both by secular law and Halacha for
both Jews and non-Jews
Therefore, the RCA hereby calls upon our
entire community, including Jews of all ages, to desist from illegally
downloading music, Jewish as well as secular, and other forms of
entertainment, in order to comply with the requirements of Halacha and
the law of the United States.
See more sources coming out against pirating software
- Ohr Somayach listing a number of reasons this would be prohibited: gezel (stealing), hasagat g'vul (reducing the developer’s rightful share of the market), mazik (inflicing monetary damage), ma’avir al da’at (violating the developer’s stipulations), dina d’malchuta dina (violating civil law)
- R Israel Schneider in the Journal of Halacha and Contemporary Society expanding on many of those concepts
- R Yirmiyohu Kaganoff on copyright specifically
- R Shlomo Brody.