Some frequently requested abbreviations are:
INST – Instant, meaning "present, current". INST is used with a date to indicate the current month. For example, "the 10th inst." means the 10th day of the current month. When the term is used in 18th or 19th century death notices, it refers to the month in which the item was published. For example, "Died on Tuesday morning, 4 inst", means that the person died Sept. 4, 1849, since the newspaper was dated Sept. 8, 1849. (from OGS Newsletter, July 1984, Page 5.)
ULT – Ultimo, meaning "in the month preceding the present".
PROX – Proximo, meaning "in or of the next month after the present". For example, "the 3d proximo" means the 3rd day of the next month.
INT – Intestate, meaning the individual died without a will.
Do – an abbreviation for "ditto".
Pro Se -- representing yourself in court without an attorney.
Check out the Researcher's Guide to American Genealogy, Pages 18-19. Rootsweb.com offers Abbreviations Found in Genealogy, a detailed listing of genealogy abbreviations.