Is there better search functionality or a plugin for the admin area of posts or custom post types, I would like to be able to search custom fields and maybe some other advanced searches from the backend only.
The Search Everything plugin works pretty well for me on the front end. I've never tried it, but you should be able to fork it and add is_admin() conditionals to all of the functions it uses. Or better yet, just allow the frond-end to be search-able too. A visitor who can find what they are looking for quickly is a happy visitor :)
WordPress › Relevanssi - A Better Search « WordPress Plugins works for both user end and backend.
Search results sorted in the order of relevance, not by date.
Fuzzy matching: match partial words, if complete words don't match.
Find documents matching either just one search term (OR query) or require all words to appear (AND query).
Search for phrases with quotes, for example "search phrase".
Create custom excerpts that show where the hit was made, with the search terms highlighted.
Highlight search terms in the documents when user clicks through search results.
Search comments, tags, categories and custom fields.
Adjust the weighting for titles, tags and comments.
Log queries, show most popular queries and recent queries with no hits.
Restrict searches to categories and tags using a hidden variable or plugin settings.
Index custom post types and custom taxonomies.
Index the contents of shortcodes.
Google-style "Did you mean?" suggestions based on successful user searches.
Automatic support for WPML multi-language plugin
Advanced filtering to help hacking the search results the way you want.
Found this one while I was searching
Turbo charge your posts admin for any custom post type with sortable filters and columns, and auto-registration of metaboxes.
Look WP GG Search! It is a powerful plugin to search everything in the backend. It supports also Custom Post types.
If you want to expand it there are a few of free plugins. It provides a easy API documentation, look at the page... My feedback to this plugin: it is very easy to use and you safe a lot of time while working with wordpress!
Edit (sorry for bad answer...): Sometimes, when you search for a special post or page in the backend of Wordpress, you search, and search, and search… Annoying… Just tap two times [G] (GG Search, what else?) and type in what you are searching for. You get a selection of entries immediately (without page reload), now just Enter and edit the post. But it does not only support posts and pages, also custom post types, comments, users,... It provides a simple action and filter API to expand your search results.