Here's a few I like that haven't been mentioned yet, not sure if any go to EU. I'll try to keep my editorializing relevant to the topic of this thread...
Marble Branch Farms: small grower specializing in compact Laeliinae hybrids but also have lots of mini species of various genera from their own and other labs; members of my local society so I purchase from them several times per year. Very friendly and approachable people.
H&R Nurseries: never ordered directly but I've gotten their
excellent seedlings from several other nurseries including the above ^. Harry occasionally posts to this forum and is both knowledgeable and nice; I highly doubt you could go wrong ordering from them. For project purposes they might be a good single-source vendor for all US participants.
Piping Rock Orchids: specialize in Paphs and Phrags incl. kovachii, but also have an interesting selection of Den, Bulbo, and other unusual species and hybrids. Not the cheapest around but quality and size of plants seem uniformly very good for the price.
Edit: have purchased twice, once mailorder and once a preorder when Glen came to speak to my society; great plants both times.
Parkside Orchids: specialize in Paphs esp. Parvisepalum spp. and hybrids, but a good selection of other genera (incl. Dens, Angraecoids, Pleurothallids, etc.) with much more than is listed on their website. Only purchased once but it was during a visit to their nursery; large, robust plants for the price and one of the cleanest greenhouse ranges of the 20-something commercial operations I've visited.
I'll try to think of some other places that particularly impressed me. Great thread idea, Ramón, should help out a lot with future projects!
--Nat