Here is a summary of Spanish grammar: el breviario de gramática, which lists and explains things like articles, nouns, verbs, adverbs and adjectives. It also has some activities on each topic.
This 24 page document can be used by anyone with little or no knowledge of Spanish as well as people who have been studying it for a while. I make it available to bloggers at no cost but if you like it please consider a donation.
Any suggestions or corrections to this document are welcome!