Observer
2 websites

Observer is free to use

http://observerapp.com/

Observer is a web analytic tool developed by Ludwig Pettersson. Observer also includes a toolbar that lets you view your analytics no matter what site you are on.

VS

Optimizely
162,903 websites

Optimizely offers both free and paid options

http://www.optimizely.com/

Simple, fast, and powerful. Optimizely is a dramatically easier way for you to improve your website through A/B testing. Create an experiment in minutes with our easy-to-use visual interface with absolutely no coding or engineering required...

Market Share by Top Websites

1 Optimizely is leading in Top 10K Sites, Top 100K Sites, Top 1M Sites and The Entire Web.

2 In terms of market share, Observer is clearly lagging behind, losing to Optimizely in all segments.

Websites Categories

1 Optimizely has better usage coverage in more websites categories. Including Games, Arts & Entertainment, Computers Electronics & Technology, Vehicles and 20 other categories.

2 Observer hasn't got a lead over Optimizely in any websites category.

Geography

1 Optimizely is leading in most countries, including United States, United Kingdom, Canada, Germany and 160 other countries.

2 Observer hasn't got a lead over Optimizely in any country.