ConCen

score another one for open source:
Calibre
for *nix, OSx, & 'doze

Calibre is an application that lets you view, convert and organize ebooks in a simple manner.

calibre is a free and open source e-book library management application developed by users of e-books for users of e-books. It has a cornucopia of features divided into the following main categories:

Library Management
E-book conversion
Syncing to e-book reader devices
Downloading news from the web and converting it into e-book form
Comprehensive e-book viewer
Content server for online access to your book collection

supported formats:
Input Formats: CBZ, CBR, CBC, CHM, DJVU, EPUB, FB2, HTML, HTMLZ, LIT, LRF, MOBI, ODT, PDF, PRC, PDB, PML, RB, RTF, SNB, TCR, TXT, TXTZ

Output Formats: AZW3, EPUB, FB2, OEB, LIT, LRF, MOBI, HTMLZ, PDB, PML, RB, PDF, RTF, SNB, TCR, TXT, TXTZ

What are the best source formats to convert?

In order of decreasing preference: LIT, MOBI, AZW, EPUB, AZW3, FB2, HTML, PRC, RTF, PDB, TXT, PDF
via softpedia:

Quote:The user interface of the program is simple to use. You can add ebooks (from a single or multiple directories) by using the file browser or the "drag and drop" function. You can also add books by entering their ISBN.

In the tag browser you can view ebooks according to their author, language, series, format, publisher, rating, as well as news, tags and identifies.

So, you can edit metadata individually or in bulk, download metadata and cover, merge ebook records, convert ebooks individually or in bulk (in the XML, CSV, EPUB, MOBI and other formats), as well as send ebooks to devices.

But you can also use the "Fetch news" feature to download news from websites and convert them into a preferred ebook format. It is possible to schedule news downloads, add a custom news source or download all scheduled news sources as well.

Calibre allows you to create libraries and toggle between them, as well as rename, delete, backup and restore libraries, connect to iTunes, start a content server, search and sort ebooks, and more.

The program uses a high amount of system resources and contains a comprehensive help file. It didn't freeze or crash during our tests. Calibre is easy to use by both rookies and experienced individuals, and provides a great solution for viewing and organizing ebooks in various formats. We strongly recommend this software.

download link:
http://calibre-ebook.com/download

This is the first and only program I've used to view non-PDF ebook formats. Not because it's better than anything else I've tried - I haven't tried anything else - but because it works well enough that I haven't found any need to find a replacement.

Others I've talked to aren't impressed with the interface, but haven't been able to go into any detail as to what changes they'd prefer when queried. It's probably more of a 'look' thing since Calibre works well in allowing one to read the ebook regardless of format, so far as I've found.

I haven't tried the convertor, but have read it's fairly solid.

you can also pick up a portable version @ the download link above.

Zang!

Good for those MOBI and EPUB format e-books that have been working their way onto the tracker as of late.

Yup. That's why I posted it. I posted it first in a torrent comment section. I'm not a big fan of proprietary formats - That's why I'll never buy a Mac or an iPhone.

I also really like its ability to take web news and put it on a reader in a non-http format.