You might not think you want or need a pen that can write underwater, on greasy paper, upside down and in temperatures from -30°F to +250°F and you probably don't. But even if you don't want to write underwater, you've probably been annoyed by a pen that stops writing when you're leaning on a wall or you're trying to make a note on a cup or laminated paper.
No need to worry about that with this pen. The pressurized ink always flows and it can handle the weather better than you. At under $20, it's great for daily use, especially since it writes for three times as long as an average pen. Refills are $9 and come in blue, black and red.
Rite in the Rain is a great brand, with waterproof notepads and other tough gear for your outdoor adventures. The star of this pen is the refill, though, which comes from the Fisher Space Pen company. You've probably heard of the Fisher Space Pen and if you haven't, it's a great story. Their pens have been on every manned space flight since the very first one in 1968.
It's also available as a clicker pen: