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
They derive from 笑う(わらう). They're the Japanese equivalent of "LOL".
www is Internet slang like lol in Japanese. It stands for warai (笑い), often used on online message boards
笑 is like www, it's another internet slang, like lol in Japanese. You will also see people adding 笑 at the end of sentences on the Internet just like the example you gave.
It is an abbreviation for warai (laugh), and it comes from gamer slang. It can be translated as "LOL."
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".