NOTE: This topic is updated whenever Microsoft releases a new theme for Windows 7. So keep checking this topic regularly to get instant updates.
NOTE 2: These themepacks also work perfectly in Windows 8 Developer Preview.
Microsoft has released a new theme "Chickens Can't Fly" for Windows 7.
Chickens Can't Fly Theme:
See what chickens are capable of! Redecorate your desktop with this free Windows 7 theme, featuring wacky images—full of deadly obstacles and delicious corn—from the Xbox LIVE game.

Download the .themepack file, double-click on it and Windows will automatically install and apply it.
BONUS TIP: If you want to enjoy wallpapers included in these theme packs, you can use 7-Zip to extract the .themepack file and you'll get a folder "DesktopBackground" containing all wallpapers.
Thanks to our friend "Rob Margel" for sharing...
How to Download More Windows 7 Themes from Microsoft Website?
You can also download other official Windows 7 themes, Desktop gadgets and wallpapers from Microsoft using following steps:
1. Right-click on Desktop and select "Personalize" option. It'll open a new window, now click on "Get more themes online" link:

2. It'll open official Windows 7 themes page in your web browser. Alternatively, you can directly open the same page using following link:
Official Windows 7 Themes Page
3. From the above mentioned webpage, you can download various free themes, gadgets and wallpapers for Windows 7.
You can also check out lots of cool Windows 7 themes in our following section:
This article was posted by VG in following section: Windows 7, Windows 7 Themes.
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Masterdevil
You can use those on windows 8 aswell just saying
PCGOD
These are NOT Microsoft released themes. These were made by someone at deviantart.com
VG
^^ No. These are official themes released by Microsoft. You can get all of them using following official page:
windows.microsoft.com/en-us/windows/downloads/personalize/themes
Its also mentioned in the above topic. :)
Shovan
Is there any way to get windows 7's themepack wallpapers in XP like
extracting those wallpapers?
VG
^^ Please check the BONUS TIP mentioned above. :)
SCBright
Halloween is not a tradition in my country, but anyway...
Jaydon Hansen
Do these themepacks work on the Windows 8 Developer Preview?
VG
^^ Yes.
Eric
where are the dinosaurs
Kevin
@ Eric - good to see I'm not the only one disappointed with the "Raptors" theme