On the surface, it sounds like you are looking for freely available UML diagrams.
There are plenty available online. If you search for "UML tutorial", you will find many tutorials, such as this one or this playlist on YouTube.
Although you asked for free, I would also like to point out an abundance of books - I usually recommend UML Distilled, but there is also UML 2.0 in a Nutshell and Learning UML 2.0 - you may be able to find these or other books at your local library if you don't want to purchase them.
So, training and example material exists, and depending on your location, may be available online or book form.
However, it also sounds like you may be looking for a complete set of UML diagrams for a system. I'm not sure that you will have any luck finding this.
Martin Fowler writes about UML Modes - UML as Sketches, UML as Notes, UML as Blueprints, and UML as a Programming Language. A complete and highly detailed set of UML models would only be produced by someone using UML as Blueprints or UML as a Programming Language. In my experiences, most people use UML as notes or sketches to communicate with other people
There are also Agile Modeling techniques. Although UML is one method, they do have some practices that would likely lead to projects not releasing UML models, such as using multiple models where the models could be an acceptance test, a user story, a "shall" statement requirement, or a free-form diagram that doesn't use any standardized modeling notation. The practice of documenting late would mean that design documents (if produced) would be produced at the end of a project to provide information to the maintenance team or next development team, rather than the models being produced before the code to guide development. Finally, there is an emphasis on a single source of information, so if a model doesn't add value, it would be discarded to avoid having to maintain it, and the source code (plus the comments and build scripts and READMEs) would likely be that single source of information.
From the standpoint of commercial software, it's not good for a company to be releasing their design material. UML is a tool used to capture the design of a software system graphically. A lot of (proprietary) information goes into that design, and without a large time commit to sanitize (probably to the point of rendering them useless) the models, they would not be suitable for public consumption. Given sufficient details about the internal workings of a software product, you not only face the business threats from competitors being able to understand how your software implements certain functions to allow them to implement those same functions or improved functions, but it can also give those looking to exploit your software insight into possible holes or vulnerabilities. There's no gain to releasing models for outside use.
From the standpoint of open-source software, the true design of the software is captured in the code. If you need help understanding the code and want a model, you can use reverse engineering tools (or use other techniques, like running the code through a debugger). Keeping released UML models in sync with the code would be time consuming for the project team, especially if they are using UML as notes or sketches and aren't using a tool capable of reverse engineering to generate them, and would only keep them away from continuing to implement the software.
I think that the short answer is that if you are looking for an open source repository of UML models, you aren't going to find it. I think you need to focus on what you are trying to accomplish.
If you're trying to learn UML, there are plenty of resources out there. You likely won't find a tutorial that uses one single system, but that's because a system that would require the use of all, or even most, of the available UML diagrams to properly document would be extremely complex. It doesn't make sense to have to learn a complex system (and the domain that it's in) at the same time as a modeling language, so make smaller problems to learn the modeling language and choose appropriate problems for different facets of the language.
If you're trying to find projects that have UML models, you probably won't find those. However, there are reverse engineering tools. Many of the good ones are commercial products. You can find some tools that can reverse engineer a limited subset of UML from code for specific languages, though. When trying to understand these systems, most people will use the software, sketch out their own diagrams and models (UML or otherwise), write test code or test cases, and use a debugger to walk through the code (see Programmers Stack Exchange 1, 2, 3, 4)