Syzygy - 3:28 pm on Aug 21, 2007 (gmt 0)

Ingram Publishing
Getty
Image Source
Jupiter Images

All of the above would be a good place to start as each offers a broad range of subject matters. Getty are the most expensive though, so be warned in advance.

As a suggestion, the British Association of Picture Libraries and Agencies (BAPLA) may prove useful as they have a searchable database of members, many of whom offer Royalty Free collections and individual images.

As Matt Probert says though, it depends on what you're looking for. There are innumerable smaller libraries out there specialising in just about anything you could think of.

Syzygy