Well, since I play DnD both at college and at home, I'm in a few groups.
First of all, my group at home consists mostly of high schoolers. I own the 3 core books, Magic of Faerun, R&R, the Psi book most of the Malhavoc stuff, Heroes of high favor, and the splats: S&F, T&B, and MotW. My brother owns the 3 core books, Magic of Faerun, FR hardcover, the BoEMs, S&F, S&S, MotW, Psionics, Spycraft, and some adventures - Of sound mind, to stand on hallowed ground, nature's fury, and the first 3 WotC path modules. We see to be the most aware of 3rd party products. Another player has the core books, psionics, and DotF. My brother fears the day he finds out about the Epic book. One guy doesn't even own dice, because his parents are anti-rpg and the last owns only the PHB, IIRC.
One of my groups at college doesn't really play 3e. Most have spent too much on previous editions and resent having to buy new things. However, some of them have PHBs, I think 2 other people have the core books, and one of those 2 has DotF, The Quint Fighter, and thieves in the forest. Of course, this info is a few months old. We mostly play other games though.
My other group played just 3e. The DM, who isn't going to back this semester, had the core books, FR, magic and monsters of FR, some of the splats, psionics, HoNS, and probably some other things. 2 people were relatively new and had nothing. The other four had at least PHBs and I think some had the other core books as well. Two of the players had some splat books - MotW, and DotF for certain.
It seems that most people tend to stick with WotC stuff. As far as I know, only my brother and one of the DnD players at college check at this sight. WotC products tend to get more exposure, and often have higher production values - full color, nice covers, etc, so they probably get lots of attention. Also, it's harder to find 3rd party stuff. My brother ended up getting some of his books at the game store in Ames, because the game stores in St. Louis aren't that great. The Hastings in Ames has more up to date d20 stuff than the Hobbytown here.
One of the good and bad things about RPGs is that people can share materials easily. At school, most people could look at the DM's or my books when they wanted to pick new spells. They don't really need to buy R&R or T&B when I show the cool spells or prestige classes they'd be interested in. We've started playing Spycraft here alot, because we don't want to run our DnD games without everyone present, but there's only book. Groups don't really need a whole lot of overlap in books, especially if characters are created at the game session. This situation is good for gamers, because it's pretty easy to get started with a relatively cheap book, borrowed dice, and an existing group. However, it's also bad for publishers of secondary materials. A copy or two of any secondary book can usually servive the entire group. Of course, it also means that some people might buy different secondary books - because the group already has a copy of splat book X, the player looking to expand his collection might get the 3rd party publisher's Z compendium. People can expand and look for new interesting things that haven't been covered, or maybe they disagree on Song and Silence's take on Bards, so they look for a 3rd party bard book. If someone in the group hadn't had S&S, they won't have know they disagreed, and might not have sought an alternative.
This is for a couple of the players in part due to poor English skills. And no, there isn't a Danish translation 
