Entries that are currently being written. (A temporary tag, remove and properly categorize when finished.)
If you are looking for high impact things to contribute to, browse popular pages and see which ones are empty or need more content. There are articles that are linked to but don't exist yet.
Some rough estimates for completeness in some of the more important categories:
Pages in category "In Progress"
The following 11 pages are in this category, out of 11 total.