Some of the phrases are explained at the Steins;Gate wiki's Real-Life References and Glossary.
Regarding the phrase「エル・プサイ・コングルゥ」(El Psy Congroo), according to Anime News Network, FutureGadgetLab is the anime's official Twitter account. According to a tweet by FutureGadgetLab, Okabe Rintarou (lab member #1) explains it as 「お前の胸の中にあるもの。それが答えだ。俺の中ではKONGROOが正しい。『俺の中では』な。《001》」, which you could translate as "It's something that's in your heart. That's my answer. When it's inside me, KONGROO is right. 'When it's inside myself,' y'know." So either the author is being intentionally obtuse (i.e., writing in character), or there is not a specific meaning for it, or the author of the tweet (whoever at the company was assigned to manage the Twitter feed by posting as the various lab members) didn't know what the meaning is. next_tales's answer given at Yahoo Japan's 知恵袋 (chiebukuro, bag full of wisdom) website also says that there is a notation stating that the phrase has no meaning in the 『ＳＴＥＩＮＳ；ＧＡＴＥ─シュタインズゲート─ 円環連鎖のウロボロス』 (Closed Chain of Ouroboros) novel. It does not seem like Japanese fans know the meaning and it is not a general meme in Japanese society. According to the ネット王子 (Netto Ouji, Net Prince) website, the phrase has no meaning but as a result of its use in this series, it has become an internet meme used by Japanese people in their online communications. So to boil that all down, 「エル・プサイ・コングルゥ」 wasn't a preexisting meme used by Steins;Gate, but it became a real meme as a result of Steins;Gate. (Though it is not a meme, the use of Dr. Pepper in Steins;Gate likewise pushed the soda into popularity in Japan [I can actually find it in a vending machine here in Hokkaido now, all thanks to Hououin Kyouma, but see these links: 1 2 3]).
I'd love to hear further answers to this thread's question.