Some services offering free downloadable karaoke songs include Karasongs and Karaoke Version. Both sites offer a wide variety of karaoke songs in a number of different karaoke formats.
Karasongs is a free karaoke service which offers hundreds of thousands of free versions of songs for download. The songs are available via a search engine, and the site also offers the option of downloads for mobile devices. Karaoke Version is a similar site, though most songs it has available for download cost a small fee. However, the site offers many free karaoke downloads as well, as well as variations on the songs such as the backing music and the instruments used.