I have experienced this first hand as well (twice) - not a very nice feeling at all.
Asides from contacting the editors, as has been mentioned. I would also inform those in your research network as to what has happened, for 2 reasons:
so they are aware of what has happened, just in case they wonder why your research has gone elsewhere.
so they are aware of the unethical behaviour of not only the offending author, but of the journal that allowed the plagiarised article to be published.
Members of my own research network let me know of both instances when I had been plagiarised (the second time was just within 48 hours of this answer). As with many academics, we have a zero tolerance for plagiarism, so we inform all in our research group for the reasons above - essentially, looking out for eachother in an academic sense.
The last point may seem harsh, but (for what very little my opinion is worth), I find that plagiarism is a deliberate and wilful act of intellectual theft - laziness and even ignorance are not valid excuses. Both the offending author and the journal that let it pass are just as guilty.