Recoll is a full text search QT based free, open source program especially made for Unix-like and Linux but it is also available for Windows and Mac systems, licensed under GPL. It provides efficient desktop full text search from single-word to arbitrarily complex boolean searches, basically it indexes the documents data (along with their compressed versions) and huge number of files then let you find quickly whatever you search for. Recoll updates its index at designed intervals (for example through Cron tasks) but if desired, the indexing task can run as a file-system monitoring daemon for real-time index updates.
The Recoll document conversion and text extraction architecture makes it extremely easy to write new filters, and many document types are supported. Indexes the contents of many document types: text, HTML, E-Mail stores of all kinds, OpenOffice.org, Microsoft Office and Office Open XML, AbiWord, KWord, Gaim, Lyx, Scribus, PDF, WordPerfect, PostScript, RTF, TeX, DVI, DjVu, MP3 and other audio file formats, JPEG and other image file formats. You can checkout complete list of features on official site.