These catalogs can be entered into any ebook reader which supports the format. Manybooks and Munseys should be entered as web pages if the option is available. These will work with (on iOS) Megareader, ShubookSE, Stanza. On Android, they should work with Aldiko, Moon+, fbReader and probably many others.
Project Gutenberg: http://m.gutenberg.org/
Baen Free Library: link has changed and will be a web page. go to baenebooks.com and take it from there.
There are other catalogs out there, but these are the most useful – and legal.