I signed up with LibraryThing recently, after seeing it mentioned around the place. If you don't know what LT is, it's a site where you can catalog(ue) your books. Free accounts are limited to 200 books, or you can get an annual or lifetime account with unlimited entries.
Most popular titles already have an entry, and the site also offers a bulk ISBN import feature which gathers data from up to three different sources.
Unfortunately, of the 3000 or so books in my house I'd only catalogued about 1300 in my own BookDB software, and of those I'd only bothered to enter ISBNs for about 300. So, it's search-add-search-add for me.
The quickest way I've found is to gather all the books I have by a given author (thank goodness my collection is alphabetical!) and then I search for that author in LT under 'Search works' on this page (NOT 'Search authors' or you just get the author name...) When the list of works comes up you can sort by title and ctrl+click all the ones you have to open their pages in new windows, where you can then click Add to library.
The people running the site are open to suggestions - in fact, they've already made one change I requested and I've only been there a few days. They'll go far with that attitude - or else they'll get purchased by your local mega media conglomerate.
Signup is really quick and easy, with no personal details required, and the site is certainly worth a look if you've ever considered making a list of your books. It's fascinating to see how many other people have copies of the books you own, and you can also leave reviews and scores out of five for any title.
As for backing up, LT offers a CSV export, and naturally I'll be adding an importer to BookDB which will read their file format.
Final thing: If you have Hal Spacejock 1, 2 or 3 you can add them to your LT from their pages below:
Hal Spacejock LT page
Hal Spacejock Second Course LT page
Hal Spacejock Just Desserts LT page
Simon Haynes is the author of the Hal Spacejock and Hal Junior series (Amazon / Smashwords / other formats)