Though this site is not geared toward Dragon Warrior Monsters 2 like others on the list, it may very well be the most important when it gets right down to it. This site features an unparalleled collection of hex editors, tile editors, table makers, assembly tools, and more.
GameFaqs houses a few noteworthy FAQs. Jimeous' walkthrough provides most of the data for Cobi's Journey and his Gameshark Codes guide is a great source for most of the game's basic ID values and a lot of important RAM addresses. There are also a few threads over at their forum that house critical data from hackers of the past. Note that DWM2_DD's Breeding Resistances guide has an entire section devoted to the storage of monster data in the RAM.
Woodus is one of very few places where you can potentially find DWM2 fans gathered. It's not exactly a hacker's paradise, but it is quite relevant to DWM2 in general. http://www.woodus.com/forums/index.php?showforum=5
This is a small site and it's not clear if it's affiliated with the larger scale DWM2 United community, but this is the only place I've seen that documents exactly how much damage the game's many skills will do.
DQShrine's old site is important because it has a few FAQs that can't be found anywhere else. A good example is Jimeous' Cobi Map Data, which was clearly made by ripping a lot of data straight out of the ROM.
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