ah, classical detectives, a reader after my own heart ! well of course the first place you should look is our very own ebook upload section, full of free public domain books lovingly hand-formatted by our members. click on "e-books" in the blue bar. you can use the handy prefix system to sort by genre ("mystery", for example). we've got several Dorothy L. Sayers which have been added recently, she's one of my personal favorites. we also have a steady stream of pulp mystery novels from Dr. Drib, so if you also like the more noir mysteries there should be plenty to keep you occupied ! if you like older classics don't miss the Sherlock Holmes omnibus. there's also a good supply of Anna Katharine Green which i've just discovered and am enjoying immensely, and Chesterton, and... well, just go take a look around !
Christie is not yet in the public domain with the exception of one of her books (which we've got here) however i believe you can get all of her books at fictionwise.com. that's a good place to check for other non-public domain authors too.
i also hope we've reached or are about to reach the tipping point and content will soon be massively available ; i'm really optimistic that ebooks are breaking out of their niche (to the degree that that is possible) with all the new devices arriving and the current ones becoming available to wider markets. in france the content situation is even more dire than for the anglophones so i really have all my fingers crossed !
oh, and welcome to the forum, by the way !