Classic Motorcycle Numbers 

Motorcycle Serial Numbers & VIN Numbers

Aermacchi VIN numbers

AMC AJS/Matchless from c1935
Engine numbers usually start with the year of production, followed by model designation, and completed with the production number of the motorcycle.
(Source - Wikipedia)

Ariel Serial Numbers
http://redhunter.com/ariel/aomcc_prefix.htm

BMW Engine & Frame Numbers
Post-1969 BMW 650 numbers

BSA Serial Numbers 
BSA Numbering System (broken 0908)
http://www.bsaoc.demon.co.uk

BSA Frame Numbers at NSW BSA Club
http://www.bsansw.org.au/scgi-bin/bsadate.cgi (broken 0901)

BSA at sunnymeadcycles (PDF)
http://www.sunnymeadcycles.com/files/BSA_ID.pdf

BSA_serial_numbers.pdf 
(to access this file, please log into the forum and then go to the downloads page at http://www.cybermotorcycle.com/forums/viewforum.php?f=37)

BSA Numbers at moto-anglaises.com
http://www.motos-anglaises.com/technique/index.htm

BSA C15 Serial Numbers
http://www.classicmotorcycles.org.uk/c15/c15_serial_numbers.htm

Francis-Barnett
http://www.users.zetnet.co.uk/Francis_Barnett/html/numbers.htm

Lambretta
http://www.scooterhelp.com/serial/serial.numbers.html

Maico
Maico Numbers at Maico Madness
Maico Numbers at off-road.com

Montesa
Montesa Numbers
Montesa Capra Numbers

Norton
Norton numbers at moto-anglaises.com
Basically the same information here:
http://www.britishspares.com/42.php
http://www.bmh.com.au/norton/modelnum.php
http://www.sunnymeadcycles.com/files/Norton_ID.pdf

NSU
NSU Max - (1) (2)

Parilla
http://www.nornet.on.ca/~tharris/parilla/models-1.html

Penton
Penton Identification - KTM, Sachs & Puch engine information
http://www.pentonmotorcycles.com/page3.html

Triumph
Triumph Serial Numbers at Bikelinks
Triumph engine & frame numbers
T120 & T140 Engine Numbers
Triumph 1905-1983 By David Gaylin
Triumph Twins
http://www.motos-anglaises.com/triumph/numero.htm

Velocettte
Velocette LE Engine & Frame Numbers
KTT engine numbers

Velosolex
Velosolex engine serial numbers

Vespa Identification
http://www.garnersclassicscooters.com/Vespa-ID.htm
http://www.scooterhelp.com/serial/serial.numbers.html

Villiers Engine Numbers