SharePoint Server 2010: It comes with Enterprise Search engine including BCS and People Search
Search Server 2010 Express: It is free Enterprise Search engine and has some limitations.
FAST Search Server 2010 for SharePoint: It provides the following Add on's:
* Item counts on the facets
* Visual previews of content
* User contexts
* Extend the core results webpart (uses XSL)
* XRank can be used for soft boost of search resutls
For IT Professionals:
* Advanced content processing out of the box
* auto extraction
* Improved admin dashboard.
* Common admin experience across all three search products.
* administration features are scriptable via PowerShell.
* Extensible search analytics reporting.
* Desktop search integration.
* Improved relevance: More parameters included in score calculation. An important new parameter is click-through rate on search results also known as popularity ranking, i.e. popular results tend to move upwards with support for customizing the relevance algorithm
* Enhanced query syntax: Support for Boolean operators like AND, OR, NOT and range operators <, >, <=, >= for search numeric ranges or date ranges.
* Wildcard search: Search for partial words using the wildcard character
* Phonetic and nickname search: Useful in people search to match similar names with different spelling.
* Faceted search with support for refining search results on any managed property configured to support refinement. Offers a very intuitive way to filter results on metadata – will make advanced search less relevant to most users.
* Query suggestions as you type
* Support for 84 languages.
* View in browser: Users can from the search results now view Microsoft Office documents directly in the Web browser without having the Office client installed. Requires the new Office Web Apps on the server.
* Sorting , Ranking and Navigation
* Keyworkds, phrases, synonyms, spell checks