Some APIs (official and otherwise) with resources

Amazon Web Services (E-Commerce Service)
• AWS Blog
• Search Google for AWS - 1.5 million hits

Google API

Yahoo Developer Net
• Yahoo Web Services Wiki

eBay API
• eBay API Code Examples
• Programming eBay Web Services with PHP 5 and Services_Ebay

Flickr Services
• Links, Docs and 3rd party applications list

43Things Web Service API

Technorati API
• Developers Wiki

Paypal
• PayPal Developer Network

National Weather Service
• Display Local Weather Forecasts with the NOAA's Web Service

Safari (Oreilly) Web Services
• Hacking Books with Safari Web Services

Atom API for Blogger
PubSub API
del.icio.us docs (very beta)
10x10 (topical keywords extracted from news sites)
GigaBlast Search Engine API

Some Sample Apps using official APIs

• Dropcash - Typekey and Paypal
• Allconsuming

• Pricenoia - Amazon
• Dealazon - Amazon
• Zuggest - Amazon

• Flickr photos on your TiVo - Flickr
• Mappr - Flickr
• Flickr Color Pickr - Flickr
• FlickReplacr - Flickr

• Whitelabel.org - Technorati
• NetNewsWire RSS Cosmos - Technorati

• Yahoo Web Services Application List (wiki)

Places to go to read more general Web 2.0 / Web As Platform observations
Jon Udell
Rael Dornfest
Paul Bausch
Stephen O'Grady
Read/Write Web

O'Reilly Webservices Books & Articles
webservices.xml.com

Possibly Useful RSS Feeds
Craigslist
Netflix
Farmer's Almanac
Upcoming.org