You see a lot of w and ww and even www in Twitter and casual chat. What does it mean? I've always thought it was わいわい but never found out. How is it pronounced?
Here's an example from Twitter
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As the others have said, the "w" stands for 笑う(わらう).
You might also see someone write "ちょwww" online. This is similar to somebody saying "ちょ~かわいい" if you've seen that line before. In this case, the ちょ is actually 超(ちょう) so "ちょwww" would translate into something like "so funny" or "very funny" compared to the standard "www" or "wwwwwwwww (ad infinitum)" which would just be "lol" and "roflmao".
Incidentally, you can also use 草 (grass) instead of 笑 or www for the same purpose. This comes directly from the fact that www resembles sprouting grass:
「www」と表現しているのが草が生えているように見えることから「草」と言われるようになりました。 Since it looks like sprouting grass, people also started to say "草" to mean "www".