Bicicletasantiguas.com.br has a pleasant online museum. The derailleurs are listed under Museu Virtual I to IV.
Bikelist.org is invaluable for searching for discussion of the truly weird!
Bikepro.com has some good pages.
Bulgier.net has lots of interesting photos.
Campybike.com sells an excellent range of copies of Campagnolo catalogues - but its site is quite informative even if you choose to buy nothing.
Campyonly.com has an attractively presented Campagnolo timeline.
Classicfuji.com has a useful (and very complete) collection of annual Fuji bike catalogues, which are useful for dating various SunTour and Shimano models.
Classiclightweights.co.uk is good on the British angle, particularly for 1950s gears.
Cyclofiend has a huge range of photos of just about every bike you could imagine.
Dave Moulton’s blog is often worth a read - it’s not really about derailleurs, but contains some pertinent comments on them amongst much else of interest.
Equus Bicycle has an eclectic collection of old catalogues.
Flikr has a fine ‘pool’ of photos. One particular photo (actually not of a derailleur) caught my eye. It’s the image of a pile of vegetable crates full of old hub-gear parts about which one viewer has commented; ‘Be still my heart’. Who says that bicycle nerdery lacks poetry and passion? (Every web site needs at least one Homeric quotation - it should be a rule for W3C compliance).
For catalogues of Japanese components (and some European components) try this fascinating site (all in Japanese).
For Huret gears of the 1960s and 1970s there is well presented Japanese site. It also features a couple of Campagnolo and Simplex models.
Greg Overton writing on cyclingutah.com has an interesting article on Zeus
Heiko has an excellent site devoted to bicycles and parts from the DDR (East Germany to us ignorant foreigners). In the list on the left hand side you click on ‘Kettenschaltung’ to get to the section devoted to rear derailleurs.
Howie Cohen has an extraordinary collection of cycling memorabilia of every conceivable date, shape, size and type. But what really captures my interest is the sense of his experience of being one of the pioneers who brought Japanese products to the US market in the 1970s. Whilst we were watching the Japanese invasion of the bicycle market from the safety of our occidental bicycle workshops, Howie Cohen was living in Japan meeting the players and doing the deals.
I can’t praise Old Ten Speed Gallery highly enough. Balm for the soul of anyone who misses the 1970s.
I want to include a link to thewashingmachinepost.net, partly (of course) because of its fine content, partly because of its name (Woodward and Bernstein meets Graeme Obree), partly because it claims to be Islay’s leading Campagnolo authority (beating off tough competition for this title) but mostly because it awards this site the prestigious ‘anorak of the century’ accolade. In the way that an Academy Award is a golden statuette and is colloquially known as an ‘Oscar’, I would propose that the ‘anorak of the century’ award should be in the form of a finely wrought golden statuette of a spotty youth with bad posture wearing a snorkel parka and holding the Ian Allan guide to aircraft numbers of the British Isles. This prestigious addition to any mantelpiece should be colloquially known as a ‘Lord Anthony’.
If you ever wondered why Youtube was invented, The Headbadge provides the answer. I love the total absence of human beings or even commentary in these films.
It’s completely off-topic but I rather like the Speedplay pedal museum.
John Forester has a usefully concise article, based on personal experience, on probicycle.com.
Jon Fischer’s magnificent Velobase.com has a huge collection of images and information. A testament to the power of open source information building.
Kinetics.org.uk has one of my favourite derailleur museums. Small but perfectly formed.
Malte Wiedenmann’s Velo-teile.eu is a very pleasant German site that has one of my favourite derailleur collections.
Panasonicbikemuseum.info has a wealth of information on Panasonic bicycles which, along with an extensive collection of catalogues, provides a handy timeline for Japanese equipment.
Retrobike.co.uk has a fine collection of scans of catalogues and magazines. Well worth a visit just for the Fat Tyre Flyer section - which proves that it’s time for wayward facial hair to re-establish itself in the world of cycling.
Robert Broderick’s Velo-Pages display what is probably the the finest collection of catalogues and magazine articles you could hope for. A testament to the power of obsessive collecting - and I mean that seriously.
The Allgemeiner Deutscher Fahrrad-Club has an excellent but very dense site with information about part numbers and dates. For example, try looking under Shimano and ‘Digest-RD’ and click the link letters beside the various models.
Tony Hadland shows photos of Arnfried Schmitz derailleur collection.
Totalbike carries an interesting article by David Diaz Blanco in three parts.
trivelox.cambrianmoons.com is Melvyn Hirst’s excellent site devoted to the Trivelox gear system - and to his uncle’s contribution to its development.
Velo-retro.com has Chuck Schmidt’s legendary Campagnolo timeline.
Velostuf.com has an interesting derailleur gallery.
Vintage-trek.com has excellent information on Shimano and Suntour date codes.
Yellowjersey.org has numerous scans of catalogues etc.