If you are a ballpoint pen lover, switch to black ink when security is important. Among water-based inks, remember that gels are the most impervious. But when you're writing checks to pay the monthly bills, only one type of ink, the kind in gel pens, has been found to be counterfeit proof to acetone or any other chemical used in "check washing." Most ballpoint and marker inks are dye based, meaning that the pigments are dissolved in the ink.Link
Update: Hanne sez, "I thought I'd put a few words in for one of my favorite inkmakers, Noodler's Ink. Specifically, their magnificent Waterproof Black ink, which bonds to the cellulose in paper fibers and cannot be washed out or dissolved out of the paper. For those concerned about the possibility of check-washing, or those who have any other forgery concerns of any sort, I can't recommend Waterproof Black highly enough."
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.