This post consolidates links previously posted by Chascomm, Kingmatic, and me in order to tidy up and centralise the information. If you find any further link please post it as a reply to this. I will then incorporate it into this post and delete yours in order to keep the information in one place.
Here are some on-line resources that may be useful for identifying the movement in your watch:
Usually the first place to look.
Excellent coverage of many manufacturers, especially Swiss.
Large eclectic collection, carefully documented.
A good selection of Timex and lots of low-grade German movements.
Makszy's view on horology - Accuracy and Development - Calibres - raszerkezetek
Another large and eclectic collection.
A great index system.
The best source for Seiko, Citizen and Orient.
They're all here.
The Electric Watch Web Site! Early Electric, Electronic and Tuning Fork Watches
An amazingly comprehensive private collection.
Electric, Electronic and Quartz
On-line copy of Pieter Doensen's legendary book.
Choose a page from the contents, then click the 'Related Photos' link at the bottom of the page.
Chronographs (in Japanese)
In the table 1st one is Landeron. Then Venus, Valjoux, Lemania, Russian (mainly Poljot), Venus again.
A few that you won't find elsewhere.
Nice big pictures.
Chronos with big pictures.
Under the Loupe/Calibres by Make - Alliance HorlogŔre
Not many pictures, but technical info such as lift angles.
Vintage ZENITH movements
Only Roamer (MST) movements - in the Technical Data section.