Hi!
To celebrate that Wikidata has received a fairly-sized donation (more Wikidata, yay!), I would like to share with you some ideas about sources for statements.
The first one, proposed by TomT0m, addresses the problem of linking one source to several statements. It could be a two column layout where on one side sources would be displayed or would be added, and on the other side the statements, to connect with those sources. Another option could be an applet to display all sources used in that item, search or add more, plus the ability to drag&drop a source onto a statement.
Then we have the issue of how to import existing sources from Wikipedia or free external sources, like Open Library, to support claims in Wikidata. The main problem is the transform of text into the corresponding items: check if an item exists, and if not create it when appropriate (author, publisher, etc). It can be done manually, but it is tedious, so some tool might help there to suggest which items to link to, and which ones to create.
Verification of web linked sources is also a major issue. How to know if the data on a web page is still current or if it has suffered changes? To address this I propose using the OKFN Annotator together with the datatype url. That would allow the user to "annotate" any portion of the web being linked, store the quotation, and then a crawler would be able to check automatically if the linked fragment has been updated. It would also highlight the text used as source when visiting the web site. There is a GsoC that aims to convert the Annotator into a Mediawiki extension to comment Wikipedia articles, and I believe it could be used to annotate texts in Wikisource (that was my original involvement in the project) and eventually linked websites in Wikidata.
https://bugzilla.wikimedia.org/show_bug.cgi?id=46440
Finally, as a reminder, when creating item-sources this might be useful to create missing items: https://bugzilla.wikimedia.org/show_bug.cgi?id=49068
And this for checking if the linked item is the right one: https://bugzilla.wikimedia.org/show_bug.cgi?id=49067
Thanks,
Micru