There are several things to consider when selecting a digital collection that will be appropriate for your research. Some things to consider:
Format - Collections can be loosely categorized into three groups. Some collections contain more than one format type.
Time period - Most collections cover a specific time period. Some collections, such as Eighteenth Century Collections Online, are very much defined by their time period. Generally speaking, there are fewer digital collections for 20th Century materials, with the exception of some newspaper archives.