While we're mildly on the topic, one of my favorite old Flash games was the Nick.com trading card game.

It wasn't really a game in the TCG sense, but more of a collecting/bartering game similar to the Grand Exchange in Runescape.

There isn't much surviving media of it since people rarely recorded game footage back then, but someone made a website of it with some screenshots:

http://www.animeexpressway.com/rugrats/ecards.htm

(Sadly, it doesn't have any screenshots of the trading screen, which was the fun part)