Several links are embedded in other pages as appropriate - here's a few more.

The ArsTechnica Guide To Silent Computers The Ars "Case & Cooling Fetish" forum has loads of info & opinion on noise-related themes.
Bit-Tech Top UK site, with some good "How-To"s on fan control and adding eye candy. And lively forums, including one dedicated to Electronics.
SilentPCReview Reviews, news and information about low-noise computers and components. Very informative forums, eg. Fans & Control
Moddin' Net UK modding site, has a set of projects for fan control every which way.
7Volts A few more ideas for a quieter computer
The Silent Personal Computer State of play, with loads of links to a broad range of topics.
General Fan Performance Guide - Basic intro to cooling fan science, covers cooling and noise. Well worth a read.
Papst Fans A good set of technical articles from the leaders in low-noise fan design.
Case Noise Reduction Fitted carpet idea. Try at your own risk.
Fan Speed Control is Cool! Fan control tutorial using Maxim's range of ics.
Rheostats Datasheets from Vishay-Sfernice, the rheostats are a bit big (12W minimum) but some advice on ratings. RS (below) stock them.
The Listening Room Novel idea - MP3 files of some popular fans. Presently limited to 60mm HS fans.
GiZzO's Fan Page Listings of some popular fan properties.
DS1621 PC-powered thermometer A temperature monitor that plugs into a spare serial port, display on task-bar. Batteries not included.
Metku Mods Some nice circuits for brightening up your life.
Component Sources
The Cooling Shop A range of fans, their Senfu LCD thermometer module is very nice, looks neat on a bay front.
Silent Fan Solutions Dorothy Bradbury sells a small selection of quiet fans, mainly Panaflo, here in the UK. Linked even though she nearly pinched my site name.
ESR Electronic Components No online store but downloadable .pdf catalogue in bite-size sections. Cheaper than Maplin, low-cost fast delivery. Recommended.
Farnell Similar to RS, but a 20 minimum order, for free delivery you need to spend 30. They stock a wide range of Papst fans and any with /2 in the code (after the N) eg 8412N/2GL has a third wire for speed monitoring. Online store, personally I find it a nightmare to search or navigate.
Futurelec Although this is a US site, they ship world-wide (from Thailand) for only $3 (about 1.70) and many items, especially the more expensive ICs, pots, heat-sinks, etc., are well below UK prices. And there's no VAT to pay on orders under 18 value. Recommended.
Kustom PC A full page of low-noise fans, and one of the few UK sources of the acclaimed Nexus Ultra-Quiet power supplies and fans.
Maplin Electronics Gone downhill fast in the last few years, expensive and a greatly reduced electronic components stock list. But as their main warehouse & trade counter is only 10 minutes away, I use them at a push. Online store.
OverClock Co. Last time I looked they had stocks of the 80mm Papst NGML fans, low noise and a decent flow.
Quasar Electronics kits No components, but a few power controllers in their kits.
Rapid Electronics Maplin for pros, no toys but a good range of quality stuff, low prices and many items that Maplin don't sell these days. On-line store, a thick catalogue at 3 or you can download it for free. Recommended.
RS (Electromail) Possibly the most extensive range of components in the UK including some Papst fans. No minimum order, but delivery is not cheap. Online store, datasheets and pictures of most items. Recommended.
Tekheads Have some Papst fans, but the medium range rather than the quieter low-range.