Digital microfilm allows you to search and download some of The National Archives' most popular records, which were previously available on microfilm. The Archives owns a large collection of microfilmed records, which they have now made more accessible by making them available online. Many of the records are indexes, which will be helpful in locating other relevant records.
These documents are free of charge to download and are delivered as large pdf files. Be aware that each download contains a whole piece, which could be up to 800 pages long. NA have not indexed the detail within the records and so you cannot search them in the same way as you can search for a medal card, for example. Instead you will need to scroll through the pdfs, much as you would when using a microfilm. With most of the items only part of the series is available online, not the whole series. Still, this collection is well worth a look at Documents Online.