Microsoft has announced two open source solutions for accessing Outlook .pst files. Read the press release here: http://www.microsoft.com/presspass/press/2010/may10/05-24PSTToolsPR.mspx
I’ve been able to find the following on Codeplex:
PSTViewTool : http://pstviewtool.codeplex.com/
PST SDK: http://pstsdk.codeplex.com/ [Thanks to Greg!]
Also, doing a search for Outlook on CodePlex offers a lot of solutions for looking at Outlook data or integrating with Outlook: http://www.codeplex.com/site/search?query=outlook
Update: Added the PST SDK.