I'm not sure how many played this, but AdventureQuest is online flash game that you could- and still can- play with your browser. It started 2002 and is still ongoing, just checked the website and there is currently 8k players. I was pretty obsessed by it when I was younger, I found the combat pretty addicting and it had lot of dumb humor in it's quests. It was also hard to play back then cause only certain amount of people could play at same time (i guess they didn't have large enough server to host everyone who wanted to play) so sometimes it put you into over 1 hour queue and sometimes it just said it's full can't play if you don't pay membership to get past the queue. That didn't really stop me though xd
Nice video on AdventureQuest