ActionVFX has released 15 free burn mark textures, to go alongside explosions in your VFX shots. The textures come as 2500 x 2500 pixels in black and white PNG's with alpha channels, and are licensed for commercial use. On ActionVFX's site you'll also find other free VFX elements, such as smoke plumes, blood mists and bullet shells.
The site was launched last year after a successful Kickstarter campaign, and aims to provide high-quality stock effects footage for VFX.
Go here to download the free burn mark textures (registration is required).