Every so often an un-official release of an operating system for any one of the various devices will surface. For those who may have just started using a Blackberry, these un-official releases are often referred to as a "Leaked OS". People working with RIM or various cell service providers routinely come in contact with these software packages while in the development process and "keep" a copy for themselves. Then they usually distribute these amongst their circle of friends or websites and it eventually makes its way all over the net.
The problem with these leaked software packages is that they are unfinished. Largely, these Operating System files get distributed before they have had a chance to be reviewed for bugs and other errors. However, when these files are distributed they are usually done so through web sites that offer support and various user input to help people get through the problems that these leaked OS'es come with. So is it worth it? Sometimes it can be. You have to remember that for all the errors and problems that may come with these packages, they also offer new features and possible fixes to a previous versions errors. This in itself gets people excited and has them actively searching for such files.
Once an operating System goes official and is released, you generally have a very clean and very operable software package. These are usually what is released by various service providers. Problem is that people generally don't like waiting for these releases. Hence the market for leaks.
There are many sites that offer Blackberry support, tips, tricks and leaks. The best by far (in my opinion) can be found at Crackberry
Crackberry offers a very wide variety of news and support along with a very good section in their forums for leaks and leak support. Give them a look-see I think you will be pleasantly suprised by what you find there. In the leaks game there are many players, but the key is finding a place that also offers support for such software packages. Because without it you will be left with unfinished software that causes more problems than it fixes. And if that is the situation you find yourself in then it defeats the purpose completely.