Custom searches allow you to setup search queries for sites you use regularly.
Find out the search URL on the site of your choice, then replace the query with {query} in curly brackets. Use the test search to ensure your search works as expected then click Save to add your new query.
Ready to create your own custom searches? Have a look at our custom search examples and this user-run website for custom searches created by Alfred users.

Explanation of fields
Search URL: This is where Alfred goes when you use the keyword that you have assigned to this custom search. {query} is what you want to search for in that specific application or website.
Display text: What is displayed in the results when you use your keyword.
Keyword: The keyboard shortcut that you want to use to call up this custom search.
Encode query using UTF8: This option helps if you're using non-standard characters. Try ticking it if your query didn't work the first time.
Examples
Open a specific website
You can use Alfred's custom search to set up keywords that will then open up specific websites that you use often.
I, for example, check Netvibes very often so I'll set up a new custom search:

Search a specific websites
You can also use the standard {query} to search specific websites, like setting up a Facebook search.

Similarly, you could use the Search URL: "http://doesfollow.com/{query}/alfredapp" to set up a search and find out who does and doesn't follow Alfredapp on Twitter.
Search within an application
Use Alfred to set up custom searches for specific applications.
You can for example search for music within Spotify:

Advanced functions
You can do so much with your custom searches! Here are a few examples:
Add a new event to your Google Calendar: Copy the following into the Search URL box: http://www.google.com/calendar/event?ctext=+{query}+&action=TEMPLATE&pprop=HowCreated%3AQUICKADD
Search your Google Calendar with the following: https://www.google.com/calendar/b/0/render?gsessionid=OK&q={query}
Related items in the Knowledge Base:
Can I tweet from Alfred?
What do the fields mean in custom search?
How can I add events to my calendar?
How can I search a specific website with Alfred?
What can I use custom searches for?
How do I find out what to put into the Seach URL field in a custom search?
What are custom searches? What are they useful for?
My favourite/preferred website isn't on the list of web searches. Can I still search it?