What does 如 in 如通常在论坛比较普遍的部分网络用语 mean?
Here, 如 can be roughly translated into "like".
However, even as a Chinese, I fail to analyze the sentence grammatically...  

The sentence can be roughly (or not-so-roughly) translated into: 

"Internet Slang" is the slang language that is popular on the Internet and may often gains popularity over the whole society. They're mostly derived from homophones, misspelled words, pictograms, and popular quotes on Internet forums. Below are some common Internet slangs on forums:
(omitted)

Well...I don't use "like" in my translation in order to convey more precise meaning.