Is it ok to send an email informing about your new paper?
I think there are two important factors to keep in mind when writing unsolicited email, which could be taken as spam by the person who receives it:


Whether you believe, in good faith, that the receiver will find the information useful.  That test is not as easy as it may seem, because what seems obvious to you (“this guy in field Y will be happy that we're developing somewhat related concepts in field X that may generate a new vision of his field”) may not be to him (“why is this X specialist writing to me to promote his recent work I don't care about?”). Messages need to be tailored, so that people can immediately see how your paper can be of interest to them.
The frequency of such messages: if you write 5 papers a year on a given topic, and send a nice informative email each time, that is a lot of mail! Consider doing this only once in a while, maybe for high-profile articles or reviews.


Now, regarding whether it's accepted practice (and common practice): I receive a dozen such emails of this nature per year, from colleagues in my field, and every time I am actually interested in their paper. I have not yet received spam of that sort, i.e. notification of new papers I couldn't care less about (I receive lots of spam job applications, on the other hand).

I also do it myself, once or twice a year: sit back, think about my recent papers, and ask myself “what colleagues do I have that may be interested in this and that?”. Usually, I then write a few different emails, depending on the specific interests of the colleagues in question, so that the email is personalized. I also use that occasion to ask questions to them , if I have any of relevance, asking their opinion on recent developments (by them, me or other groups) in our field.