I have tried reading books on my computer screen before, and I have been disappointed. Mostly, I have realized that scrolling sucks. Given that I have already paid to download a few technical books as PDF, I needed a reader that could make the PDF experience easier than Preview or Adobe.

(psst: I've heard good things about the Kindle, though I have not extensively used one myself. [affiliate link])

EDIT: If you're looking to create ePUB files, check out the beautiful and open source Sigil.

I'm learning to develop for iPhone, so I found two books from Pragmatic Programmers, Cocoa Programming Quick Start and iPhone SDK Development.

Following this, you'll get a screenshot of each reader, and a bit about why I would / wouldn't use it.

Here's Preview:
[caption id="attachment_864" align="alignnone" width="266" caption="Preview"][/caption]

After having gone through the other apps in this list, Preview is actually not too bad for reading technical books, but not my first choice.

I had heard Andy Ihnatko talking about Stanza. I think he likes it to synch books between his iPhone and his laptop. I'm pretty sure you can also buy books wirelessly from your iPhone inside this app.
[caption id="attachment_866" align="alignnone" width="300" caption="stanza"][/caption]

The problem here is that this app is designed for reading story books - tons and tons of text, no fancy formatting. In fact, this app strips out all of the formatting and fonts, to the point that it makes no sense for tech manuals.

I saw Tofu on another site reviewing Mac eBook readers.
[caption id="attachment_865" align="alignnone" width="300" caption="Tofu"][/caption]

In the description, there was a line "Tofu is different". In fact, Tofu works almost exactly like Stanza (at least for my purposes). While Tofu did keep more of the fonts and formatting, Tofu still wants to break the layout that the original publishers created, which makes this the wrong choice for programming books.

Then I discovered my saving grace: Skim - and it's open source!
[caption id="attachment_863" align="alignnone" width="300" caption="Skim"][/caption]

It may be because it is created by programmers scratching their own itches, or maybe because it is not geared specifically for mobile devices or story books, but something just felt right about Skim. I really like the ability to highlight text and leave notes. Everything you highlight now becomes a bookmark you can jump back to for future reference. This seems like an app I'll keep open while I'm coding to go back and refer to examples and explanations. More or less, exactly what I was looking for.

It also has pretty good search functions - when it finds a word you're looking for, it circles it in red. You also have tools to manually circle text, draw a box, underline or strike through text, or draw arrows to help you annotate graphics.

There is a full screen mode and a presentation mode, as well as a neat feature called the reading bar - basically, a line-by-line bookmark to let you remember where you left off.

I've said the most about Skim because that's my pick. It's free, Mac-only, and open source.

A final note: I also tried an application called eReader Pro. This seemed to be tied to an online bookstore, and had zero support for PDF. Any tech book I've seen comes as a PDF, so steer clear of this one if you're a programmer.