Some of the most valuable resources for researchers of early Plymouth County families are now available at FamilySearch.org - page images of the Plymouth Colony records of probates and deeds.
The original Plymouth Colony records were, in 1818, separated, repaired, and rebound as 23 volumes (plus two index volumes). Among the records of most interest to researchers are the four volumes of probate records (long referred to as "Old Colony Wills") and the six volumes of land records (long referred to as "Old Colony Deeds").
The probate records, in one set of images, are at:
Parts of those records have been transcribed or abstracted and published, but the page images are the only complete versions of those volumes available online.
Seventeenth-century writing can be difficult to read for those of us in the twenty-first century. I would suggest that newcomers to seventeenth-century hands should arm themselves with a good guide to colonial handwriting, and practice reading the page images side-by-side with quality published transcriptions such as those by George Ernest Bowman in Mayflower Descendant, in order to gain proficiency in accurately reading those images.
I have compiled a guide to the four volumes of probate records, which serves as an index to the page images of those records and includes references to published transcriptions and abstracts of those records. Also included are references to published probate records not found in the page images. One example is the 1621 will of William Mullins, which is not found in the Colony records, but which survives in the records of the Prerogative Court of Canterbury, now in the National Archives in London. Transcriptions of that will have been published in at least three places, as cited in my guide, which also includes citations to transcriptions and abstracts of other probate records not found in the Colony records.
The guide (as well as my earlier index to the published volumes of Colony records) is available at: