Affiliate Marketing (can be combined with any web idea below)
Adsense
Amazon (The web's most popular and successful Affiliate Program)
Site Scout (Self-Serve Advertising Platform)

Sell premium plugins for popular software
Wordpress

eCommerce
Drop shipping

Build an online community
Facebook page
Run a forum

Content Aggregation
Scraping data from various sources to re-organize, analyze or present in a better format.
News in various niches
Trading data
Social feeds (Facebook/Twitter/Instagram)

Publish technical books
Sell on amazon kindle
Pragmatic programmers

Create an automated online course
Driving lessons
Coding courses

Automated Trading (Finance)
Bitcoin Arbitraging
Trading Bots (https://news.ycombinator.com/ite...)

Finding ideas and niches to build apps:

Finding problems to solve
Read Rework or Getting Real by DHH from 37 Signals.
http://howtofindsaasideas.com/
Pick up a hobby that you truly enjoy other than programming, find problems in that area that can be solved with technology.

Good references for Passive income:
5 Steps to $5,000 in Monthly Recurring Revenue
Four hour work week
Ask HN: Passive income ideas?
Automatemysmallbusiness
Rich Dad Poor Dad

http://www.quora.com/What-are-the-top-ways-for-a-web-mobile-programmer-to-build-passive-income-streams