A few years back I purchased Houdini 2, but I was curious abut this Houdini 4 PRO that ChessOK is offering for free so I downloaded the .rar file, but didn't have anything to unzip it with.
I tried WinZip, but my Webroot anti-virus kept giving me warnings, so I was a little hesitant about unzipping and running WinZIP. After doing some reading up on it at the Webroot site, I think WinZIP is OK, but they didn't like it because it has a lot of invasive crap that, while annoying, is not really dangerous; that's why Webroot gives the warning. At least I think that's what the article was getting at. Anyway, somebody recommended a free program called 7-zip, so that's what I downloaded and installed to unzip Houdini 4 PRO.
I was suspicious of this free Houdini 4 engine...how can it be free when everybody is selling it? ChessOK is a legit site...I've bought from them before and while I'm not a fan of some of their products, they do have excellent customer service.
I ran match at 3 minutes per game pitting my Houdini 2 vs. Houdini 4 PRO downloaded from ChessOK. The result: Houdini 2 won with +6 -1 =2. If it's the real Houdini 4 PRO, how can that be? I decided to try a longer (15 minute) game. The result (I only ran one game) was a long, boring draw. Honestly, after the trouncing it took I am skeptical that this is the real Houdini 4 PRO engine. In fact I think it performed so badly I deleted it.
While on the subject of engines, a lot of players are looking for "human-like" play from engines. BlackMamba is a multi-core engine for Windows (32 and 64bit), Linux (32 and 64bit) and Android. The engine is very strong and supposedly was written to make (sound, as opposed to just giving away material, I guess) human-like sacrifices. I'm not sure how one tells human moves from computer moves in this situation, but below is a G10 Black Mamba vs Stockfish 6. If anyone is looking for a human-like engine you may want to try out Black Mamba.