Below is a list of commonly used file extensions.  You can find a somewhat more complete list here of file^* types and extensions. These lists were gathered from the very extensive Wikipedia's  List of File Formats page.

Archive and compressed

The most common compression files to the standard user are .zip and .rar. However, there are many more.

• 7z 7-Zip compressed file
• cab - Microsoft Cabinet
• gzip (.gz) - Compressed file
• RAR Rar Archive (.rar), for multiple file archive (rar to .r01-.r99 to s01 and so on)
• sit - StuffIt (Macintosh)
• tar
• .tar.gz, .tgz (gzipped tar file)
• zip

Document

These files store formatted text.

• DOC - Microsoft Word
• DOT - Microsoft Word Template
• DOTX - Microsoft Word 2007 Template
• HTML - HyperText Markup Language (.html, .htm)
• HWP - Haansoft Hangul Word Processor document
• ODT - OpenDocument Text document
• OTT - OpenDocument text document template
• RTF - Rich Text Format
• TXT - ASCII or Unicode raw text file
• WPD - WordPerfect document
• WPS - Microsoft Works document
• WPT - Microsoft Works Template
• WRI - Microsoft Write
• XHTML (.xhtml, .xht)
• XML - eXtensible Markup Language - broader than just documents - OpenOffice employs XML

Graphics

• BMP - Microsoft Windows Bitmap formatted image
• GIF - CompuServe's Graphics Interchange Format
• JPEG, JFIF (.jpg or .jpeg) - a lossy image format widely used to display photographic images.
• PNG - Portable Network Graphic (lossless, recommended for display and edition of graphic images)
• PSD, PDD - Adobe Photoshop Drawing
• PSP - Paint Shop Pro image
• TIFF (.tif or .tiff) Tagged Image File Format (usually lossless, but many variants exist, including lossy ones)

Page description language

• PDF
• PostScript (.ps, .ps .gz)

• Configurations, Metadata
  • CSS

Presentation

• KEY, KEYNOTE - Apple Keynote Presentation
• ODP - OpenDocument presentation
• OTP - OpenDocument presentation template
• PPS - Microsoft PowerPoint Show
• PPT - Microsoft PowerPoint Presentation

Audio

Lossless audio

• Uncompressed
  • AIFF
  • AU
  • WAV - Microsoft Wave
• Compressed
  • M4A - Apple Lossless (M4A)
  • WMA - Windows Media Audio 9 Lossless (WMA)

Lossy audio

• MP2 (MPEG Layer 2)
• MP3 (MPEG Layer 3) - most common audio format these days
• Vorbis (Ogg project, free and similar in principle to MP3)
• WMA - Windows Media Audio (.WMA)
• AAC (.m4a, .mp4, .m4p, .aac) - Advanced Audio Coding (usually in an MPEG-4 container)
• RealAudio (RA, RM)
• VOX - Dialogic ADPCM Low Sample Rate Digitized Voice (VOX)

Playlist formats

• ASX - Advanced Stream Redirector (.asx)
• M3U
• PLS
• RAM - Real Audio Metafile For Real Audio files only.

Spreadsheet

• 123 - Lotus 1-2-3
• CSV - Comma-Separated Values
• ODS - OpenDocument spreadsheet
• OTS - OpenDocument spreadsheet template
• TAB - tab delimited columns; also TSV (Tab-Separated Values)
• WKS - Microsoft Works
• XLS - Microsoft Excel worksheet sheet
• XLT - Microsoft Excel worksheet template

Tabulated data

• tab
• CSV - Comma-separated values
• dif (accessible by many spreadsheet applications)

Video

Main article: video file formats.

• 3GP - the most common video format for cell phones
• GIF - Animated GIF(simple animation; until recently often avoided because of patent problems)
• ASF (ASF is a shell, which enables any form of compression to be used; MPEG-4 is common. Video in ASF-containers is also called Windows Media Video (WMV))
• AVI - (AVI is a shell, which enables any form of compression to be used; MPEG-1 and a variant of MPEG-4 are common)
• FLV (*.flv) A video file encoded to run in a flash animation.
• M1V MPEG-1 Video file
• M2V MPEG-2 Video file
• SWF - Macromedia Flash (.swf for viewing,
• FLA - Macromedia Flash for producing) (
• MNG (mainly simple animation containing PNG and JPEG objects, often somewhat more complex than animated GIF)
• MOV (QuickTime, a container format, which enables any form of compression to be used; Sorenson codec is the most common)
• MPEG (.mpeg, .mpg, .mpe)

• MPEG-4, shortened "MP4", a popular video format most often used for Sony's PlayStation Portable and Apple's iPod.

• NSV Nullsoft Streaming Video is a media container designed for streaming video content over the internet.
• RM - RealMedia
• Xvid

Video Editing & Production formats

• MSWMM - Windows Movie Maker Project file
• PPJ - Adobe Premiere Pro Video Editing File

Webpage

• Static
  • HTML - (.html, .htm) - HyperText Markup Language
  • XHTML - (.xhtml, .xht) - eXtensible HyperText Markup Language
  • XML - (.xml)
• Dynamically generated
  • ASP - (.asp) - Microsoft Active Server Page
  • ASPX - (.aspx) - Microsoft Active Server Page. NET
  • CFM - (.cfm) - ColdFusion
  • CGI - (.cgi)
  • JSP - (.jsp) JavaServer Pages
  • PL - Perl (.pl)
  • PHP - (.php, .php?, .phtml) - ? is version number (previously abbreviated Personal Home Page, later changed to PHP: Hypertext Preprocessor)
  • SSI - (.shtml) - HTML with Server Side Includes

XML, markup language and other web standards-based file formats

• EML - (.eml) - File format used by several desktop^* email clients
• RSS - (.rss, .xml) - Syndication file format
• Atom - (.atom, .xml) - Another syndication file format