Adding sources to your individual entries in a genealogical database can be time consuming and tedious. Some of the online family tree type programs are trying to streamline the task by automating parts of the process. This seems to be spreading rapidly throughout the genealogical online community.
The first site I noticed with a semi-automatic sourcing capability was WeRelate.org. This wiki program for genealogy files incorporated a separate wiki page for every resource in the Family History Library Catalog. The results was that to add a source to your online file, all you had to do was add a link to that resource's wiki page. Unfortunately, it sounds a lot simpler than it was in practice, but the convenience of having the entire Family History Library catalog available to attach to your individual ancestors was impressive.
I then realized that Family Tree Maker, the program from Ancestry.com, had developed the ability to link to your Family Tree on Ancestry.com. The Family Trees on Ancestry.com had added a "leaf" connection to possible sources. This automatic lookup system was remarkably good at finding related documents in their collections. With the link between Family Tree Maker and by adding your entire file to Family Tree on Ancestry.com, you could essentially automate a huge portion of the task of adding sources to your file. If for any reason you did not want to keep all your information online, you could use Family Tree Maker to look up the sources directly and add them to your local database. You could also export your file via GEDCOM to another database, if you wished. Over time, I have found this feature of Ancestry.com/Family Tree Maker to be more and more valuable.
Recently, with the introduction of FamilySearch.org's Family Tree, they added a feature called Sourebox, that allows you to add links to sources from FamilySearch.org's Historical Record Collections almost automatically by adding them to a temporary holding list called the Sourcebox. The individual sources can then be attached to the ancestors in your file by simply clicking on a link. Once the sources are attached, they can be deleted from the Sourcebox. FamilySearch.org's Family Tree may also add the ability to link to sources outside of their own databases.
Now, MyHeritage.com has added the ability, through 1000Memories's app Shoebox, to add scanned documents directly to your Shoebox and ultimately directly to your online MyHeritage.com family tree.
I think there is a now a definite pattern here of the integration of online sources with genealogical databases. But the serious question all this raises is whether or not you data, including all your sources, will be locked up in one company's format? If you stop paying Ancestry.com or MyHeritage.com, what happens to your collection of scanned images or sources?