You can find it here. Though this place is full of opinions, and I don't guarantee you'll find the correct answer based on the majority of opinions here.
Spidey is right in that the best way to find what your looking for is to try searching for the previous threads on it. There are numerous that inevitably end up in a bitter debate which this one may also - I will make my post and watch the entertainment. To answer your question my favorite script is phpld - I don't use the free version and never have so my opinion is based on the paid version I use. The reasons I like it is there are many mods available and it is not too difficult to find good developers willing to work on special needs you have. I also like the support and people over at the phpld forums. The best advice for you if you are looking for a free script is to read up on the features of the various scripts and since you are wanting the free version download the top two or three and try them out for a couple days each. Just messing around with the script. Once you find the one that works best for you develop a site based off that script.
To give an honest answer, i use the paid version of phpLD but ive not used the free version but i hear that the free version of phpLD is good. I know what i like and i guess you will find what you like too.
Might have been a wise decision to do a search, save you a lot of time and hopefully you'll get to see more than just one script as there are many good ones available.
try phpld's free version, but its better to use the paid version, cause it offers more features, mods etc. There are a few other scripts (See one at JamieG's signature) are also available, compare the features, and get one.