The biggest difference between Western and Japanese fan communities is that the latter can actually sell their goods. America in general is a much more litigious society, but they're willing to turn a blind eye to fan creations as long as the material in question isn't for profit. On the other hand, the Japanese have a whole economy driven by copyright infringement. This mostly results in huge amounts of porn featuring popular characters, as well as unofficial video game spinoffs (see Duo Princess, an action game featuring the characters of Square's Threads of Fate, or Elepaper Action, a beat-em-up starring the girls from Read or Die.) Naturally, this applies to remixed music CDs, too. OCRemix releases its arrangements for free, because they have to, but that doesn't apply to Japanese remix circles. (There have been exceptions with Western groups selling albums, like Project Majestic Mix and its Final Fantasy arrangements, or OneUp Studios releases of some Yasunori Mitsuda and Sega tribute albums awhile back, but they actually went through the proper legal channels to license the tracks.)
Much of the actual content is similar, of course. If you wade through all of the various doujin circles, you'll be overwhelmed by Mega Man, Chrono Trigger, and countless arrangements of Final Fantasy battle themes. It is a good way to find new and interesting stuff outside of the video game music mainstream (yes, such a thing exists!), which is largely what makes the doujin video game music scene so appealing. You'll find that a lot of them have huge appreciation for Romancing SaGa, a series all but disdained outside the country, along with tributes to companies like Sega, Cave or Falcom, or themed albums based on various genres like fighters or shooters. There are also an ABSURD amout of Touhou CDs - it's quite boggling how popular the series is. (I haven't heard much of the arranged music, but the OSTs are quite good.)
Doujin albums are also a lot cheaper than official music CDs, often wavering between 1000-1500 yen, as opposed to the 2500-3000 yen usually commanded in record stores for official CDs. Furthermore, the days of extensive officially sanctioned arranged music CDs of the 90s are long gone, with only an occasional one trickling out, so not only do doujin CDs fill a niche, but they do it quite cheaply too. (There was apparently a case where a Mega Man rock CD called Megalomania was pulled around the time that Capcom was releasing their own similarly styled albums, but I don't know the details or what became of it. I actually do own the album, though, and will probably review it in the future.)
The problem is, finding these CDs for Westerners is often quite difficult. Most regular albums can be found at Amazon.jp, but doujin albums aren't even sold at regular record stores, instead inhabiting conventions like Comiket or anime good stores like Toranoana, who refuse to ship overseas to dirty, ignorant foreigners. So, much like all doujin goods, anyone outside of Japan usually just resorts to pirating them. Occasionally you can find them at places like http://www.paletweb.com/, http://www.himeyashop.com/, or http://www.otaku.com/, and eBay can occasionally yield them too, especially from the seller champ_des_pins. But their selection is spotty and shipping can be killer at some of them. A CD that should be $10 quickly jumps up to $25 when markup and shipping is accounted for.
Now that we've got some background, please look forward to some doujin music CD reviews in the future! Places like http://www.vgmdb.net/ occasionally have tracklists and such, but I've never really found any place that reviewed them, so I'm hoping to fill in the gap, at least a little bit.