BIGR = British Isles Genealogical Register. Published by FFHS.
BT = Bishop's Transcript. Copy of parish register lodged annually.
BTW = (By the way)
CRO = County Record Office. Local archive where most U.K. registers are kept.
EIC = East India Company.
EST = Europaeische Stammtafeln - Stammtafeln zu Geschichte der Europaeischen
Staaten - edit. Baron Freytag von Loringhoven / Detlev Schwennicke - publ. J.A. Stargardt, Marburg-an-der-Lahn, Germany.
FFHS = Federation of Family History Societies.
FHC = Family History Centre. Run by the LDS (see below).
FHS = Family History Society. Usually based on a county or region.
FTM = Family Tree Magazine. Published monthly in the UK.
GENUKI = Genealogy UK + Ireland. News group and www pages.
GM = Gentlemans' Magazine. Ceased publication in 1868.
GM = Genealogists' Magazine. Quarterly journal of the SOG (see below).
GMA = Gemmingen-Massenbach Archives
GOONS = Guild of One Name Studies. For the xxxx family worldwide.
GRD = Genealogical Research Directory. Published annually.
GRH = General Register House, Scotland.
GRO = General Register Office. Open at St.Catherine's House, London, U.K.
HMSO = Her Majesty's Stationery Office. U.K. Government publisher.
HTH = (hope this helps).
IGI = International Genealogical Index. From the LDS (see below).
IHGS = Institute for Heraldic and Genealogical Studies. Based in Kent, U.K.
IMHO (in my humble opinion).
IOL = India Office Library, part of the British Library. London, U.K.
IPM = Inquisition Post Mortem. What you had after you had gone.
IRC = International Reply Coupon. To be included if you can't use an SAE (see below)
ISBN = International Standard Book Number. Loved by all booksellers!
LDS = Church of Jesus Christ of the Latter Day Saints. The Mormons.
MI = Monumental Inscription. What is on a gravestone.
NRH = New Register House, Scotland.
OPCS = Office of Population, Censuses & Surveys. U.K. Government department.
OPR = Old (or original) Parish Register. Usually in Scotland.
OS = Ordnance Survey. Mapmakers for the UK.
PCC = Prerogative Court of Canterbury, England. Church court for proving wills.
PCY = Prerogative Court of York, England. For North of England.
PPR = Principal Probate Registry. Somerset House, London. Wills after 1858.
PRO = Public Record Office. Main repositories in Kew and Chancery Lane, London, U.K.
RAF = Royal Air Force.
RN = Royal Navy.
RCHM = Royal Commission on Historical Manuscripts.
SAE = Stamped addressed envelope. Included for replies.
SOG = Society of Genealogists. Library and Office in London.
SRO = Scottish Record Office - Edinburgh.
SSAE = Stamped self-addressed envelope.
UK = United Kingdom.
WWW = (World Wide Web)
(P) = Prince / Principality / Kingdom
(D) = Duke / Duchy / Herzog
(M) = Marquis / Marquisate / Margrave
(C) = Count / County / Graf
(E) = Earl / Earldom
(V) = Viscount / Viscountcy
(B) = Baron / Barony / Freiherr
(Bt) = Baronet / Baronetcy
* = born
+ = died
c. = (circa) about
ante = before
post = after
Much of the above list of acronyms has been compiled from suggestions supplied by Barney Tyrwhitt-Drake [firstname.lastname@example.org], the Bucks Genealogical Society, & various other contributors. Additions are welcome.