Adobe Systems is to buy internet services firm Omniture for $1.8 billion, the company said Tuesday. The deal will mark the biggest acquisition by Adobe since its $4.3-billion purchase of Macromedia in 2005.

Omniture, based in Utah, makes software that allows firms to analyze the use of their internet sites and help them maximize advertising and retail opportunities. Adobe, whose key products include Photoshop and Flash, will pay $21.50 a share for