Wiktionary is the product of a collaborative volunteer effort. New contributors often wonder where to start. Below are projects that could use some help. Many are big but have well-developed techniques, so they are a great for volunteers who need to begin with a model. (Talking of "models" - on a smaller scale, contributors can skim the Wiktionary:Style guide to see styles that have been developed and a few ready-made blocks of headings, etc., that can be copied to save a lot of time when creating new entries.)
Articles to write
Requested articles — These words do not have entries yet. You can start whole new pages.
Frequency lists — Ongoing projects to create various frequency counts, some of all the words in all the books in Project Gutenberg. Some for the most common words in Wikipedia. One list contains words that are simply the most common words, another (10-part list) identifies words that were not defined in Wiktionary, as of 2 October 2005.
Wanted pages — The Wanted pages is a useful link; try changing red links to blue.
Collaboration of the week — These existing entries need review before they feed into the translation project.
Check translations - Many translations of English entries are listed without a specific sense. When the English entry is completed or a specific sense is added, contributors use {{ttbc}} or {{ttbc-top}} to list the entry in Category:Check translations. If you speak English and another language, please take a shot at clarifying those translations.
Blends - As with the retronyms on Wikipedia, there is a list of portmanteaus (many of which are actually blends rather than true portmanteaus) too, many of which don't yet have articles in Wiktionary. Again, this is a list of many interesting words with ready-made definitions.
The standard etymology to use here is of the form:
{{blend|XXX|YYY}}
where XXX and YYY are the words combined to form the blend; for example, for smog}}, {{blend|smoke|fog}} gives “Blend of smoke}} and fog}}”.
Pronunciations that differ in UK and US English (and other varieties) - A list of these words is given in Wikipedia. Add pronunciations for these only if you are thoroughly familiar with how to transcribe UK and US pronunciation using the IPA and SAMPA pronunciation schemes.
Wikipedia terminology lists - Wikipedia has various glossaries and terminology lists and appendices that could be a helpful source of definitions of specific sets of words. One such list is in w:Musical terminology. Partial lists also reside at w:Category:Glossaries and w:List of glossaries. Many are good candidates for manual importation and editing to check the definitions and make them Wiktionary-ready. We can also help both projects by flagging other Wikipedia word lists with these categories as we find them.