Suggest text classifier training datasets Which freely available datasets can I use to train a text classifier?

We are trying to enhance our users engagement by recommending the most related content for him, so we thought If we classified our content based on a predefined bag of words we can recommend to him engaging content by getting his feedback on random number of posts already classified before.

We can use this info to recommend for him pulses labeled with those classes. But we found If we used a predefined bag of words not related to our content the feature vector will be full of zeros, also categories may be not relevant to our content. so for those reasons we tried another solution that will be clustering our content not classifying it.

Thanks :)
Some standard datasets for text classification are the 20-News group, Reuters (with 8 and 52 classes) and WebKb. You can find all of them here.