In Adobe Album you could show all photos that had not been tagged with any keywords yet.
Or type in kids/christmas/1994 and then click for all the pictures to show up without those three keywords. Is there a way to do this in iPhoto?
The easiest way is to create a Smart Album where "keyword is none." If you then assign a keyword to those photos, they will disappear from that smart album. You can do this until no photos remain untagged, if that is your goal.
Filtering with the keyword tool is also very flexible. Use the keywords pane (open with the little key icon in the lower left of the iPhoto window). Click a keyword in your list to filter the library by that word (included words will be blue). Option-click a keyword to exclude it from the filter (excluded words will be red). You can do any combination of clicks and option-clicks to create very specific filters for your library. There is also a Preference setting which determines if selecting 2 keywords will filter the library to photos marked by one or the other word (Match any), or if it will restrict the filter to photos marked by both words (Match all). To find all photos with no keyword, you can Option-click all the keywords in your list. I tried it with both Preference settings ( +match any+ and +match all+ ) and got the same result each time. To add new keywords, open the Preferences > Keywords, click the little + and type a new word in the blank. Be careful not to begin or end the keyword with a space, or problems can crop up later. (But spaces inside a multi-word keyword, such as "Hot Springs" are okay.)
The calendar tool is very flexible, as well. You can filter your library by a year, month, week, or date. Option-click a month, week, or date to find the photos for every year in your library. For example, Option-click your birthday to find photos taken on your birthday each year.
