Aldiko and FBReaderJ are similar in that they only support DRM-free (not encrypted) ebooks. I prefer this kind of app, all things being equal, since I always remove any encryption as soon as I buy an ebook. For a review, see
FBReader on EVO 4G Phone--Quickie Review. These are free apps, so I suggest you find an ePub (say) you like from the
upload forum or from
feedbooks and try them out. If you are a science fiction fan, I suggest
My Own Kind Of Freedom by Steven Brust - which is only available as an ebook.
There are ebook readers tied to particular formats and encryption schemes and ebook sites.
Kindle for Android is "coming soon", and has been reported to be bundled with the US Dell Streak. Kobo and B&N also have readers (B&N in beta I think).
None of these reader apps support PDFs, but there are dedicated PDF apps out there. One of the most often recommended is RepliGo, which is not free. Adobe has a basic free PDF app, but it needs Android 2.1.