Stanza. It took a while for an iPad version to appear, but it's been here now also for a while, and since I only have three or four major ebook reading apps on the ol' pad, naturally I decided I needed one more.
It's a good one though. It's the first I've seen (compared to Kindle, iBooks, and Nook) which lets you choose from *all* the fonts in the system. The text size adjustment is virtually stepless, which I love, most apps have too crude steps. It has stepless variation of margins. The other apps don't even *have* such an adjustment. It has in-app access to many online book sources, paid and free. (Oh, and amazingly, the app itself is free too.)
I've only used it a bit so far, but right now, it's a clear Recommended.