I subscribe to lots of newsletters, and just skim through most of them.
However, I always read through these 3 newsletters as soon as I receive them.

1. The Daily Reckoning.
Bill Bonner and his crew make finance interesting. Their very readable analysis of global events, together with a wicked sense of humour makes this my favorite newsletter.
I’ve learned more from this free newsletter than I ever did in my university economics courses.

2. The Monday Morning Memo.
Roy H. Williams of the Wizard Academy writes a weekly newsletter which is always unpredictable and inspiring.
I thoroughly recommend going through his archive of previous posts.
The one which had the most impact on me was written back in 2006. It’s called “Live Your Crowded hour”, and is about intensity - and taking a chance. Read it at
http://www.mondaymorningmemo.com/?ShowMe=ThisMemo&MemoID=1646
Audio versions of the newsletters are also available as podcasts. (And they’re all on my iPod.)

3. Boogie Jack - Almost a Newsletter
Dennis Gaskill’s award winning newsletter is about about web design, and also life in general. He writes with a sense of humour, and provides a lot of great website related information.