Quick look at Popular Extensions
 

.AI-> Adobe Illustrator
.ANI->Annimated Cursor
.ANS->ANSI text
.API->Application Program Interface
.ART->An art file
.ASC->ASCII text file
.ASM->Assembly Source Code
.ASP->Active Server Pages
.AVI->AVI movie format
.BAK->Backup copy
.BAS->Basic program file
.BAT->Batch file
.BI->Binary file
.BIN->Driver
.BK->Backup copy
.BMP->Bitmap
.C->C program file
.CAB->Microsoft compressed format
.CDR->CorelDraw
.CFG->Configuration
.CGI->Common Gateway Interface
.CHK->Checkdisk
.CLP->Clipboard
.COB->Command file
.COM->Command file
.CPL->Control Panel
.CPP->C++ program file
.CSS->Cascading Style Sheet
.CTL->Control file
.CUR->Cursor Image
.DAT->DATA
.DBC->Visual FaxPro Database
.DBF->Database file - dBASE
.DLL->Dynamic Link Library
.DOC->Document file
.DPR->Delphi Project file
.DRV->Driver
.EML->Electronic Mail
.EXE->Executable file
.FNT->Font
.FOR->FORTRAN program file
.FRM->Form design- Visual basic
.GIF->Graphics Interchange Format
.HLP->Help
.HST->Host
.HTML/HTM->Hyper Text Markup Language, files read by WWW browsers
.ICO->Icon
.IDE->Integrated Development Environment
.IMG->Image
.INI->Initialization
.IOB->Imagine file
.ISU->Netscape file
.JPEG/JPG->Image file name
.LNK->Shortcut file
.MDB->Microsoft Access Database
.MID->MIDI
.MIME->Encoding format used by Multipurpose Internet Mail Extension
.MOOV/.MOV/.MOVIE->Quicktime Movie
.MP3->Mpeg Layer III compressed audio
.MPE/MPEG->Movie or animation file
.MSP->Microsoft Paint
.NET->Network file
.OBJ->Object module
.PAS->Pescal program file
.PL->Pearl Script
.PM->PageMaker
.PPS->PowerPoint Slide Show
.PPT->PowerPoint Presentation
.PRJ->Project file
.PRN->Print file
.PSD->Photoshop Document
.REG->Registration file
.RTF->Microsoft Text and Graphics
.SAV->Save Game
.SHTML->HTML file with embedded server side includes(SSIs)
.SND->Sound file
.SRC->File used for .INI files configuration settings
.SYS->System file
.TAR->Tape ARchive
.TAR.GZ->Combined archive of TAR and then gzipped
.TAR.Z/TAZ->Combined archive of TAR and then zipped
.TGZ->Another name for .tar.z and .tar.gz
.TMP->Temporary files
.TXT->ASCII text file
.VBP->Visual Basic Project
.WIN->Windows Backup file
.WLR->VRML page
.WMF->Windows Meta file
.WP->Word Perfect
.XLS->Excel worksheet
.XM->MOD music format
.Z->UNIX compressed file
.ZIP->Compressed file using PKZIP