The word pair is right up there (::points to the blog post title::): stationary and stationery. They sound exactly the same, and sadly the latter has fallen into disuse to the point where some people don’t even know the word anymore.
Something stationary is stable, unmoving. Stable and stationary both have an A in them. That might help you remember. (I know, there’s also an E in stable. However, the problem syllable isn’t “sta,” it’s “ary.”)
Stationery is paper goods for writing letters or note cards. Some people include writing implements in the category, since you can’t write a letter on stationery without a writing implement. You remember letters, don’t you? We wrote those before we had email. (Some of us still do it.) Stationery shops can still be found if you hunt hard enough; of course, if you’re not in the mood for hoofing it, you can always shop online. The words letter, pen, and envelope all contain the letter E, and so does stationery. That’s always been my mnemonic for it. (You’ll note I use words that do NOT also contain an A. That would be madness.)
You hold your stationery stationary with your hand while you write your letter.