(NOTE: this is part of the “Software I like” series)
The MobiPocket Reader is the software I use for reading ebooks in my Series 60 cell phones (the 6630 and the N-Gage). (there are also Palm, PocketPC, UIQ, Windows SmartPhone and Windows desktop versions)
Contrary to what you may think, reading ebooks in your mobile phone is perfectly fine, and fully readable without tiring out your eyes - not as pleasant, certainly, as a real book made of paper, but, on the other hand, you tend to bring your cell phone anywhere and have it with you all the time. And it can easily hold 10-20 books or more, with a mere 128MB card. When I go on holidays, I usually bring a bag of books; this summer, I just brought my 6630, with the “Autumn” series by David Moody, plus, of course, the Zombie Survival Guide (don’t leave home without it!) 
MobiPocket don’t just make the reader - they’re also an ebook store. I’ve bought many books there - they have a lot of fantasy and science fiction, and prices are usually lower than those of the “dead trees” versions.
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I use Repligo that has an excellent viewer. Mobipocket seems better for its content (medical ebooks and dictionaries).
Mobipocket Reader is the best ebook software IMO, especially for PDAs and Smartphones.