I really dig O'Reilly's Safari. Not that I'm in love with reading technical books online - I'd rather have the book in front of me to take to another room and sit down and grok a subject completely instead of sitting at my computer. But the selection is pretty damn great. And the ability to swap out books means that I can bop from one neato tech to another without having to lay down a bunch of cash. Having 10 credits a month means up to 10 books that focus on what I'm working on at the time. It's always changing, and I really like the flexibility.
Also, I'm sort of trapped here in Spain where the markup on English language books is pretty freakin' amazing. If, after reading it online, I decide it's worth having in my collection, I'll go buy it and pay the ransom. If not, I didn't waste the money. That's awesome.
Also, you get some books ahead of everyone else. Scott over at the Fuzzy Blog was talking to his editor today and found out that O'Reilly's Blogging is going on sale in three weeks. I just went over it this weekend on Safari. Cool. I was even able to give Scott a heads up that some of the links in the book that point to his site weren't working.
In case you're interested, here's my subscriptions for this month: