Pylons

3aims Web Development
Arcadia and Ricono - Italian literary agency. Site is backed by a lightweight, i18n-enabled CMS built on Pylons 0.9.6.1 using elixir for modelling, babel for i18n and mako for templates.
A Swift Half - UK pub searching and reviews.
AuthKit
Bel-EPA - personal site of Graham Higgins, uses Pylons with a native XML database (eXist) and semantic web RDF (rdflib).
BitTorrent - (yes that BitTorrent)
Blackberry Tracker Pylons site that enables you to track GPS-enabled Blackberry Phones.
Blockparty NYC - Allows users in NYC to add and find blockparties taking place in their neighborhoods.
byCycle.org - allows users to find the best bike routes in a couple select cities (so far)
http://btbytes.com - personal site of Pradeep Gowda. Uses pylons with docutils,Mako on a NGINX server
Car.gr - Largest online car classifieds website in Greece.
Clinical Indications - In conjunction with NHS Innovations South East
Codepad - A pastebin that runs code in a variety of languages (including Python, of course)
debexpo - Debian package repository used for mentors.debian.net
debshots (screenshots.debian.net) - storing screenshots of Debian applications with a spice of jQuery
Developer Community (Ukranian) - Russian/Ukranian web portal for developers.
Driverfall - World's biggest driver resource, currently in test drive
Entic.net - VPS server/Pylons hosting - Get up to 50% off just for Pylons developers - email us!
FreeLDAP.org - a free LDAP service for address books and more
FireEye, Inc.
FormBuild
Fotobuch-XXL
Foxmarks
freebase - a open database of the world's knowledge, currently in beta
Gazest - Pylons powered wiki application (demo site)
Idée Labs - Idee's technology showcase, focusing on visual search. Front-end is powered by Pylons.
IM Feeds - Delivers RSS/ Atom updates to your instant messenger
IncidentNews - Information on current and historic oil spills/hazardous material spills in US waters, by NOAA's Emergency Response Division (US government site)
indiconews - Citizen journalism website, currently only the facebook application is using pylons but a new version of the site is being launched shortly (mid november 08) with the entire site ported from php to pylons.
JeugdKosk - Youth section (in Dutch) of the Chess Club of Ostend (Belgium).
LosoweHaslo - A simple random password generator with a Polish interface
Lunartics - Meet the Universe's most quirky and colourful oddballs!
MadForIT
Market Shares Australia - In-depth current and historic stock market analysis and portfolio management for the Australian stock market
Matisse & Jack's - Natural foods online store for Matisse & Jack's products
MochiAds - Ad network for Flash games
Na-szlaku.pl Polish, a portal about mountains, expeditions, trekking and climbing. Uses Pylons, sqlalchemy and Mako.
PDFTextOnline - PDF Text Extraction in Your Browser
Parachute
Popego - Know your (Digital) self.
PrideLife - A website which accompanies the UK based pridelife magazine. Fully developed with Pylons including a custom made CMS.
PylonsHQ - This website
Project HQ - An open source project management system. The site runs Drupal, but the app itself is written in Pylons (see the online demo of the project).
Random Idea - Webdesign and development portfolio site showcasing the talents of Random Idea
Reddit - a collaborative news site (announcement)
Snowtide Informatics Systems, Inc.
TRTF - The Russian Technology Fund (venture capital investment opportunities in Russia)
The Microshare Project - Pylons XUL demo
This One Next - Book, music and film recommendations
TradeVibes - A platform for sharing, discussing, and evaluating information about new startup companies.
Two Peas In A Bucket - The internet's #1 online scrapbooking retailer and community. They're migrating from ASP to Pylons which currently drives the entire e-commerce front end.
URM Exercise class distribution and management at a University in Germany. Students register with their preferences for the classes and get distributed in an optimal fashion. Site is in German.
What Should I Read Next? - Book recommendations
Walter Cruz - Artigos
Jaworznicki Portal Społecznościowy - Polish, local portal for city Jaworzno.
Webhosting.pl - Polish, portal about internet technology
Joblint - Software job board
Pure Expressions Photography - Photography studio

Links not currently working:
Cheeseboard - A Cheeseshop mirroring system (10/14/2007)
IMDBTube - All IMDB records and related YouTube videos, currently in development (8/11/2008)