I exchanged my Canon D30 to 20D at 2005-03-15. The change was done because of following reasons: better autofocus, better ergonomics and better high ISO performance. Below is my little comparison between D30 and 20D based on actual usage not reading other reviews.

20D pros:

• Autofocus is much faster and more accurate.
• Ergonomics are much better, for example focus point selecting joystick is very good.
• Build quality is better, camera feels much better on hand.
• High ISO performance is much better.
• 8Mpix is more than 3Mpix, but at my use when only publishing photos to internet and printing always A4 size, this isn't very big benefit. Naturally slight cropping can be done and still print at A4.
• Better battery life using same BP-511 batteries.

20D cons:

• File size is much larger, before my 1GB MicroDrive was enough for half day travelling in nature but now it's full in couple hours.
• Setting ISO value is much more difficult than in D30 [in D30 you can set the button inside back command dial to activate ISO adjustment, in 20D you have to press one of the buttons in front of the top LCD-screen and I haven't yet learned (= I will not learn never) to find that button without moving my eye from viewfinder]

Equipment links (Link to parent category)

Canon EOS D30

DLC photography - D30 tips
DLC photography - Chuck Westfall's tips
Fred Miranda - D30 FAQ
Reviews

Dpreview
Luminous Landscape 1
Luminous Landscape 2
Rob Galbraith 1
Rob Galbraith 2
Steve's digicams
Photo.net
Digital Outback Photo
Imaging Resource

Flash photography with Canon EOS cameras
Canon EOS FAQ

Pentax

Bojidar Dimitrov's K-Mount Equipment Page
PDML - Pentax-Discuss Mailing List

Lenses

Van Walree Photography - Optics (many good articles)
Canon TS-E lenses

Tilt and shift lenses (regarding all TS-E lenses)
TS-E 45/2.8 on the-digital-picrure.com
TS-E 24/3.5 on the-digital-picrure.com
TS-E 90/2.8 on the-digital-picrure.com
Sports with TS-E lenses by Vincent Laforet for the NY Times
Fred Miranda - how to correct CA when shift used
Cambridge in colour - Shift
Cambridge in colour - Tilt

Carl Zeiss lenses

digLloyd Zeiss ZF lenses
Zeiss.com - Carl Zeiss Distagon T* 35mm f/2.0
Zeiss.com - Carl Zeiss Makro-Planar T* 100mm f/2.0
16-9.net - Test of 28mm lenses including CZ 28/2.8
Pebble Place - Carl Zeiss Planar T* 2/100
Pebble Place - Carl Zeiss Macro-Sonnar T* 2.8/100 N
SLR Lens Review - Contax Sonnar T* 2.8/135mm C/Y
SLR Lens Review - Contax Sonnar T* 2.8/85mm C/Y
SLR Lens Review - Contax Planar T* 1.7/50mm C/Y
Pebble Place - Contax Sonnar T* 2.8/135mm C/Y
Some example pictures of Zeiss lenses
pinciuc.com - samples from various lenses

Nikon PC-Nikkor lenses

16-9.net - Test of 35mm shift lenses
mir.com.my - PC-Nikkor 28mm f/3.5
mir.com.my - PC-Nikkor 35mm f/2.8

Adapters

Pebble Place - C/Y adapters

Cambridge in colour - Understanding camera lenses
Photo.net - Lens tutorial

Canon EOS 20D

Dpreview 20D review
Luminous Landscape part 1
Luminous Landscape part 2
Luminous Landscape field test
Photo.net review

Canon EOS 1D Mark III

The-Digital-Picture.com
Imaging Resource
Ken Rockwell
Rob Galbraith
Photography-on-the.net