The Rijksmuseum in Amsterdam is really in a league of its own as far as official museum websites go.

Most major museums have the ability to view their collections online (except the Museo del Prado which either does not have the facility or has it very well-hidden) but they are often only in low-res small versions – see the Guggenheim for an example.

The National Gallery in London has a slightly different approach. With their online gallery you have the option of looking at a small-ish .jpg version of pictures or using a java-based zoom feature, which lets you get in close but can only see a small portion of the picture at a time, which has a watermark all over it. 

By contrast the Rijksmuseum lets you see absolutely huge .jpgs of their collection, without any sort of interference.  What makes it really great is the Rijkswidget.  This is a Yahoo! widget which displays a work from the collection on your desktop. You can flip it over to read a bit of information about the day's picture and jump from there to the main website to view a large high-res version or find out even more about it.

So while all the other museums seem to want to treat me like a potential thief, obscuring my view of paintings, the Rijksmuseum is actively encouraging me to see and learn more.  it is an example to all the others, and a small consolation for the museum itself having only a fraction of its collection on display due to the massive renovations.