List is still incomplete
Lots of people are asking for Aeron chairs: I'm going to suggest some alternatives even if they're not in your desired price range.
I'm going to divide this into three sections: High-end, mid range and low-end.
High-end
#1 Baron Okamura: One of the best chairs in the market
Available in UK @Think-Furniture
Starting price: Unknown

#2 Orangebox Spina chair, rebranded from Itoki Design's SP chair
Available in UK, US, Au, Japan
Starting price: ~$700US
Feature: Active lumbar support, when you sit in the chair the lumbar comes toward you: youtube: watch?v=OqdhTOryLyc
elastomer back
#3 ON Wilkhahn 3 awards silver/gold last year @Neocon ... 3d movement
Available in UK @Think-Furniture
Price: Unknown

#4 Teknion Contessa made in Canada
Available worldwide, also known as Okamura Contessa (Okamura rebrand)
Starting price: ~$700US

#5 Alpha 80 Executive Duorest
Available worldwide
Starting price: ~$900US
Website

#6 Nightingale CXO (My current chair) made in Canada
Features: gel pad for lumbar, memory foam (seat, lumbar, headrest), dual mesh back
Available worldwide, HealthyBack
Starting price: ~$950US

#7 Haworth Zody
Feature: Dual lumbar support
Available worldwide, Healthyback
Starting price: ~$1000US

#8 Steelcase Leap
Feature: Live lumbar support
Available worldwide, Healthyback
Starting price: $870US

#9 Knoll Generation
Feature: Flexibility
Available on Amazon for $900US

#10 Knoll life
Starting price: $1000US
Available at Sit4less
#11 Allsteel #19
Available at Think-Furniture (UK)
Review: Think-Furniture

#12 Allsteel Acuity
Review: http://jhopthoughts.blogspot.com/2009/12/review-allsteel-acuity-chair.html
Starting price: $750US
#13 Herman Miller Embody
Feature: Spine structure
Available worldwide
Starting price: $1000US
#14 Herman Miller Aeron
Available worldwide
Staring price : $800US (loaded)

#15 Humanscale Liberty
Personal review: Not recommended
Starting price: $750US
Available at TheHumanSolution
#16 Humanscale Freedom
Personal review: Not recommended, plastic arms break easily, cannot adjust recline
Available at TheHumanSolution
Starting price: $899US

Mid range
#17 Raynor Ergohuman
Feature: All mesh
Available at Ergohuman.com, amazon, etc...
Starting price: $579US
#18 Herman Miller Mirra (Successor of the Aeron)
Available worldwide: Sit4less
Starting price: $640US

#19 Humanscale's Diffrient World Chair (Successor of the Liberty from Humanscale)
Available worldwide: TheHumanSolution
Starting price: $629US
Notice: This chair will be released this fall

#20 Steelcase Think
Available at TheHumanSolution
Starting price: $649US

#21 Haworth Very Task Chair
Available at Sit4less or
Starting price: $680US

#22 Knoll Chadwick
Available at The Human Solution, All modern, Ergonomic Chair pro, etc..
Feature: Elastometric back
Starting price: $540US

#23 Allseating Cpod
Feature: 3d movement
Available at Sitbetter

Low-end
#24 Herman Miller SAYL
Available at Sit4less
Starting price: $399US

#25 SwingSeat
Info: I'm listing it here even if it's not recognized as a "real" ergo chair, but some people do find it comfortable
Available at SwingSeat, GotYourBack, etc...
Starting price: $495US

Stool/Sit to stand chair
#26 Via Swopper
Features: Bounce up/down, moves with you
Review: It forces you to keep your back straight in order to keep balance
Available worldwide, TheHumanSolution
Starting price: $650US

#27 HAG Capisco
Features: Saddle type chair, moves with you
Review: Apparently one of the best to build/keep healthy posture
Available worldwide
Misc.
I didn't include the following, but also check your local availability for:
#28 Mitos chair from Interstuhl
Available at Think-Furniture

#29 Teknion Amicus
Used price: ~$150-200US

#30 Silver from Interstuhl

This list is constantly updated
Thanks for looking
Lots of people are asking for Aeron chairs: I'm going to suggest some alternatives even if they're not in your desired price range.
I'm going to divide this into three sections: High-end, mid range and low-end.
High-end
#1 Baron Okamura: One of the best chairs in the market
Available in UK @Think-Furniture
Starting price: Unknown

#2 Orangebox Spina chair, rebranded from Itoki Design's SP chair
Available in UK, US, Au, Japan
Starting price: ~$700US
Feature: Active lumbar support, when you sit in the chair the lumbar comes toward you: youtube: watch?v=OqdhTOryLyc
elastomer back


#3 ON Wilkhahn 3 awards silver/gold last year @Neocon ... 3d movement
Available in UK @Think-Furniture
Price: Unknown
#4 Teknion Contessa made in Canada
Available worldwide, also known as Okamura Contessa (Okamura rebrand)
Starting price: ~$700US

#5 Alpha 80 Executive Duorest
Available worldwide
Starting price: ~$900US
Website
#6 Nightingale CXO (My current chair) made in Canada
Features: gel pad for lumbar, memory foam (seat, lumbar, headrest), dual mesh back
Available worldwide, HealthyBack
Starting price: ~$950US
#7 Haworth Zody
Feature: Dual lumbar support
Available worldwide, Healthyback
Starting price: ~$1000US
#8 Steelcase Leap
Feature: Live lumbar support
Available worldwide, Healthyback
Starting price: $870US
#9 Knoll Generation
Feature: Flexibility
Available on Amazon for $900US
#10 Knoll life
Starting price: $1000US
Available at Sit4less

#11 Allsteel #19
Available at Think-Furniture (UK)
Review: Think-Furniture
#12 Allsteel Acuity
Review: http://jhopthoughts.blogspot.com/2009/12/review-allsteel-acuity-chair.html
Starting price: $750US

#13 Herman Miller Embody
Feature: Spine structure
Available worldwide
Starting price: $1000US

#14 Herman Miller Aeron
Available worldwide
Staring price : $800US (loaded)
#15 Humanscale Liberty
Personal review: Not recommended
Starting price: $750US
Available at TheHumanSolution
#16 Humanscale Freedom
Personal review: Not recommended, plastic arms break easily, cannot adjust recline
Available at TheHumanSolution
Starting price: $899US
Mid range
#17 Raynor Ergohuman
Feature: All mesh
Available at Ergohuman.com, amazon, etc...
Starting price: $579US
#18 Herman Miller Mirra (Successor of the Aeron)
Available worldwide: Sit4less
Starting price: $640US
#19 Humanscale's Diffrient World Chair (Successor of the Liberty from Humanscale)
Available worldwide: TheHumanSolution
Starting price: $629US
Notice: This chair will be released this fall
#20 Steelcase Think
Available at TheHumanSolution
Starting price: $649US
#21 Haworth Very Task Chair
Available at Sit4less or
Starting price: $680US
#22 Knoll Chadwick
Available at The Human Solution, All modern, Ergonomic Chair pro, etc..
Feature: Elastometric back
Starting price: $540US
#23 Allseating Cpod
Feature: 3d movement
Available at Sitbetter
Low-end
#24 Herman Miller SAYL
Available at Sit4less
Starting price: $399US
#25 SwingSeat
Info: I'm listing it here even if it's not recognized as a "real" ergo chair, but some people do find it comfortable
Available at SwingSeat, GotYourBack, etc...
Starting price: $495US
Stool/Sit to stand chair
#26 Via Swopper
Features: Bounce up/down, moves with you
Review: It forces you to keep your back straight in order to keep balance
Available worldwide, TheHumanSolution
Starting price: $650US
#27 HAG Capisco
Features: Saddle type chair, moves with you
Review: Apparently one of the best to build/keep healthy posture
Available worldwide

Misc.
I didn't include the following, but also check your local availability for:
#28 Mitos chair from Interstuhl
Available at Think-Furniture
#29 Teknion Amicus
Used price: ~$150-200US
#30 Silver from Interstuhl
This list is constantly updated
Thanks for looking