To search for a topic in a specific namespace, precede the topic with that namespace's prefix, followed by a colon. If the topic has a page with that name, pressing Enter will take you right to it. Otherwise, a list of relevant search results will appear.
For example, one way to get a listing of articles on ancient Greece is to type Category:Ancient Greece in the search box. To go to the portal for the Ancient Greeks, you would type: Portal:Ancient Greece.
The namespace prefixes are: Category:, Portal:, File:, User:, Talk:, Wikipedia:, Help:, Book:, Draft:, and Template:.
You can also search in sister projects by using the full name of the sister project, or its abbreviation (e.g.- Wiktionary: or wikt:).