I'd like to be able to take any piece of Russian text and have the stress marks automatically added. I think this would be a useful tool for learners but I can't find a site that does this. Is there one out there? If there isn't I'd even consider creating one and would welcome any ideas about how it could be implemented.
Try russiangram.com. It's free and does exactly what you want: add stress marks and dots above the letter Ё.
Not an online application, still:
Программа RussAcc предназначена для озвучивания текстов, создания аудиокниг (в формате wav, mp3, amr, aac + плейлист m3u), расстановки ударений (Nicolai, Alyona, Loquendo) в русских текстах (отсюда и ее название - сокращенное Russian Accents), удобного чтения книг с экрана. Можно использовать RussAcc как редактор текста, конвертор FB2 в TXT. Заяц русак, как известно, способен быстро ударять по барабану, RussAcc же очень быстро расставляет ударения в текстах.
Just in case: I'm not affilicated with them, but used the tool some time ago, seems to be quite nice (except for the edge cases).
http://www.grammatica.eu/ is probably the best software of this sort, and the upcoming second vesion should make it even better. It's quite expensive, but I can highly recommend it.
I have a stress marker on my personal website. It searches for words here and matches the words given with regex. It does not matter if the word is conjugated or not, but keep in mind it cannot know whether to use i.e.
облакА. And before searching you need to make it lowercase and strip the dots off of
I think you should mention your goal in your post. For instance, if you would like to read something with stress marks, there are plenty of texts available for you--should you be adept at combing through the internet and resources published by Russians for foreigners, or even for children. If you would like to mark something up with stress marks, then this brings me to my third concern. If you are worried about missing the stress marks since you are still developing your Russian, then you may want to consider the fact that this problem will hammer itself out with time, and not just through reading--you may need to include a triangulated strategy of listening to things, a chart you make or have of some of the unexpected shifts in stress/accent (or one you've found), and a text along the same level where these words will likely appear. However, if you feel like your Russian is on a nice level where you more or less know the stress marks, and are still anxious about a few exceptions, it's time to take off the training wheels and ride with the big boys.