The Light My Fire Swedish FireKnife is very useful. The blade is a fixed hardened Sandvik stainless steel blade, and the FireStarter is a twist-lock handled steel made from magnesium alloy. Need a knife? Bam! Need a fire too? BAM!
The FireStarter even works when wet, and will last approximately 3,000 strikes. That's a lot of built campfires. Also, the knife/firestarter combo has received a high 4.7/5 stars from nearly 400 reviewers at Amazon, where the knife is a nifty $26.49.