I've always been a big fan of Palm PDAs. I've owned quite a few different models, all of which were quite close to the 3x5" form. But none more perfectly embodied it than the Sony Clie TH-55. I stopped using it about a year ago when I had to start using a Blackberry for work, but I still keep it charged up to play with sometimes, and I miss using it daily!
Here's a picture of the Clie in its case.

I should mention that this case, by Sena, was the best PDA case ever: not too much bulk, beautiful leather, and a tidy magnetic closure: no ugly loop and snap disrupting the clean edges. And on the inside, handy pocket for business cards and extra memory sticks. It also came with a small mirror, which I removed. This case was actually designed for a different PDA model, and the openings didn't perfectly match up with the TH55's buttons, but the leather was thin enough that I could still use everything perfectly except the camera.

The Clie was so nice and thin, very easy to slip in a pocket:

I used it for everything-- there are so many nifty little Palm OS applications that go far beyond the basic calendar/contact/task uses.

And here is that lovely full screen calendar view-- with no more appointments, sadly! I used to have lots in there, but with the small font option, it was easy to assess a whole week at a glance.

I wish Palm had been a smarter company and not let Blackberry surpass them. If I could, I would still be using this Clie-- my Blackberry may be the only compatible option for my work email, but it's not 3x5!