Astronomical Data Center (ADC): With a Service for Amateur Astronomers

"ADC for Amateur Astronomers"
NASA

"Special Note: The World Wide Web contains many, many sites for Astronomy, many of which may be of interest to amateur astronomers. The ADC primarily serves the professional astronomy community. However, amateurs can find many of the ADC's catalogs useful for their hobby. This page includes information on using ADC resources for amateur astronomy and links to selected external resources that may be of interest...."

This is a pretty good set of link pages. I found one (external) link that was no longer in use, but the others worked.

Although it's mostly about data - numbers, tables, that sort of thing - there are links to some quite decent astronomical images.