Day One is a small Mac-iPad-iPhone for journaling. It is a very simple app that elegantly reminds you to make journal entries at time intervals you select throughout the day. It handles pictures very well (taking pictures with an iPhone/iPad or using images from the camera roll). I have tried dozens of these kinds of apps over the years including just adapting standard word processing programs.
This is the only journaling app that I have ever been able to motivate myself to use on a regular basis.
It strikes me that this could also be a very good program as a lab notebook or data analysis notebook in that major issues can be typed in with automatic timelines, pictures, etc. While it does not have a built in sketching routine, you can always do a rough sketch on a piece of paper and photograph it as part of the entry with an iPhone or iPad camera.
UPDATE: In December, Apple chose Day One as their app of the year for 2012.
UPDATE 2: February 6, 2013. I like this app more than ever. I frequently use the synching between Mac and iDevices and it works very well and seamlessly. It is possible to import old photographs and (if the photo has a standard date entry) the photograph will automatically land on the correct date page of the journal.