I've started using it lately.  I've been very impressed.  I am shocked to see people using wikipedia at work when we have, no shit, access to
ClinicalKey
UpToDate
Micromedex
AccessMedicine
Lexicomp
and a online access to a zillion journals for free.
It's funny, I remember carrying a backpack to work so that I had
Washington Manual
Merck Manual
The ICU Book
A nurse's IV compatibility book----Everything is compatible in anesthesia, I came to learn.
Calculator----we shot cardiac outputs via ice saline--thermodilution---sent a mixed venous and and abg--
we then calculated all of the parameters with a calculator using a laminated card on our ID badge with all the formulas. Then we hand wrote them on the flowsheet.

The point of this thread is to mention some online resources and learn about the others that you guys use.

Now, here is the question---if you are coming out of pocket for a subscription, which one would you choose or have chosen, and tell us your specialty.  

Evan.

A couple hospitals I'm affiliated with pay for subscriptions for me. I use UpToDate and Lexicomp exclusively.

Emergency medicine

Doctor, I will have to call you out.  I know you to be an Emergency Medicine physician.  I dare you to deny that you have at least contemplated using a 1998 ed PDR (there are usually 12 or 14 old PDRs laying about) to beat a patient, colleague or nurse to death.  
How dare you, sir, call a PDR worthless.

OH, and for a dislocated shoulder, sometimes, when the doc is very busy, we would gingerly put the patient face down on a stretcher with the arm dangling. We would then put an old PDR or Harrison's Internal Medicine textbook in their hand. It would sometimes fatigue those shoulder muscles and pop the shoulder back in.

I've been damn impressed with both.  UpToDate is awesome in that they update their stuff regularly and it is really useful.

But how do you download it for when the internet is down?

You don't I would imagine. I have the clinical key app and UpToDate app on my phone. I guess with cellular service I would still be OK
I have some textbooks on my phone, ipad as well.  

Our computers at work, including those in the OR, have an enormous number of resources on them.
But, people still pull up wikipedia.

We have had a lot of problems with the net going down causing the emr to lock up. I dream of downloading a full reference that updates every night and F the net.
We did just install fiber and it gotten better but in the land of manana everything gets fixed tomorrow.
We do have work arounds but its a PIA.

Same here

I was able to get uptodate and clinical,key on my phone
Through work subscription
It works with no connectivity.  
I guess it updates every so often