There are many ideas already on the web about how to use authentic materials in the classroom! Here are some ideas which cover English, Spanish, French, and more! I'm fond of the idea of using comic strips in classrooms. They're short, offer pictoral cues to what the text may be, and involve humor. It's also something that students of all levels can enjoy and create one of their own.
Choosing the proper text can be difficult. This article provides key factors to look for when selecting a possible text, including both student interest and the ability to use it to teach grammar concepts.
As with most teaching theories, there are pros and cons with using authentic materials in a language classroom. The students can gain a lot of knowledge using this method but it does come at the cost of being more work for the teacher.
The Ohio Department of Education created a list of authentic resources for each language sorted by theme. Each resource has a short description of what each is about. While some languages have more resources than others (Spanish has much more than Latin/Greek), they are all classroom appropriate and suggestions are given to some about how to use them.
This is another aggregation of materials suitable for language classrooms with many music, TV, and radio options to choose from. Personally, I like the International Children's Digital Library as it offers low level native texts. For intermediate to advanced students, this can also be intertwined with a project to create their own children's book.
The newest edition of the book which I learned many teaching techniques from, the Teacher's Handbook offers many great pedagogical theories and examples of how to use them in the classroom. The main idea it offers when selecting authentic texts for classroom use is to "Edit the task, not the text." In other words, using higher level texts on lower level students is fine, but alter what they do with it.
What are authentic materials? This Cambridge blog post defines them as untouched materials used by native speakers for native speakers. It also claims that texts which have been tweaked in some way as being only "semi-authentic."