If you're on the road during the holidays and looking for a good museum, Visit Museums (free) is a nice app to have on your iPhone.
Using the GPS in your iPhone, you'll get a list of nearby museums, along with contact information and addresses. There are links to museum websites, and a map to help get you there. You can also browse any location in the world for museums of note.
The app found some obscure museums in my area, and some other places I looked, so I think its database is reasonably complete. What it lacks is any detailed information on the museums, being satisfied to take you to the museum website. While the map is handy, the app doesn't give directions. I'd like to see you be able to click on a museum address and get directions from Apple Maps.
There isn't a lot of competition from similar apps. Most museum apps are produced by specific museums, like the Smithsonian. I did find one general app that is similar, called Museums of the World, but it had a much smaller listing of museums to visit. It skipped my nearest big city, Phoenix, completely.
Where To? (US$2.99), which is a navigation app, has an even more extensive list of museums than Visit Museums, and it connects to Apple Maps and other navigation apps like Navigon and Google.
Visit Museums requires iOS 4.0 or greater. It's not a universal app, so it's best used on an iPhone or an iPod touch.
I liked the idea behind the app, but wish it had more information and map integration.