http://itunes.apple.com/us/app/prague-travel-guide-mtrip/id4…
http://itunes.apple.com/us/app/trondheim-guide/id435101762?m…
http://itunes.apple.com/us/app/singapore-travel-guide-mtrip/…
http://itunes.apple.com/us/app/rome-travel-guide-mtrip/id377…
http://itunes.apple.com/au/app/new-york-travel-guide-mtrip/i…
http://itunes.apple.com/us/app/istanbul-guide-mtrip/id389764…
Free travel guides, seems to get decent reviews, but haven't really gone through them in detail yet.
Update:
The AU store seems to have expired, the US Store is still free except NYC.
Update 2:
The old ones have expired.
The following are now free (thanks to ironsword & petestrash)
http://itunes.apple.com/au/app/hong-kong-travel-guide-mtrip/…
http://itunes.apple.com/au/app/venice-travel-guide-mtrip/id3…
http://itunes.apple.com/au/app/stockholm-travel-guide-mtrip/…
http://itunes.apple.com/au/app/barcelona-guide-mtrip/id37713…
http://itunes.apple.com/au/app/munich-guide-mtrip/id38977918…
I loved using the London and Paris ones on my recent holiday there last month. It was really useful to help find places and work out an order of things to do. In particular, having an offline map to refer to was particularly useful.
Even if you do not get a data plan, your iPhone 4 will be able to pick up your GPS location so finding out where you are and how to get to places is easy.