If you frequent Japanese message boards or comment sections, or perhaps follow some Japanese Twitter accounts or have some J Facebook friends, you've probably seen roman "w"s scattered in with the kana from time to time. And you if you do frequent those sites, then probably know what they mean. If you don't and/or you're not, however, here's a quick explanation for you.
Like English and I suspect may other languages, Japanese has its own evolving "internet language." I suppose this crosses into text messaging realm as well, but I'm not sure if there's some term that encompasses both. In any case, "w" is kind of the Japanese version of "lol."
You see, the word for laughing or laughter is 「笑う」. Sometimes you'll see 「笑」 thrown into posts or comments to denote laughing. This can be further shorted to "w" for "warai."
Here's an example from the wild that I lifted from a Japanese friend's Facebook. The context is that he posted a picture of himself with his hair dyed blond, and a friend commented:
(Who's that? lol Nice to meet youuuuu)
One interesting additional note is that more "w"s can be added to indicated a longer or more intense laugh. Kind of like changing it to a "lmfao," with room for more degrees in between, I suppose.