OhDon'tForget.com
It does only one thing, but it does it oh, so well. On the home page, just type in a cell phone number and a brief message ("Pick up dry cleaning") and this site sends it out as a text message to that phone on the date and at the time you specify. The site detects your time zone so you don't have to include that, nor do you have to even register or look at any ads. (Recipients will be charged a regular text-messaging fee.) You'll need to be brief, as there's a 160-character limit on messages, although soon users will be able to ramble on longer, and Oh, Don't Forget... will transmit the message in parts so that it all gets through, the site's creator Jason Stirman says. The service works with all U.S. carriers, a few select Canadian ones and a few in Europe and Japan, but you don't have to know your recipient's carrier to send a message. (If you changed carriers but saved your old number, you must take one extra simple step: see Add Your Number.)
Another way to nag your spouse: Teleflip's flipOut service, which lets you email a text message of up to 120 characters to anybody's phone by sending it to yourfriendsnumber@teleflip.com. Your email appears as a text message, and the reply turns up as an email message in your inbox. TeleFlip has another service called FlipMail, which lets you read your email on any cell phone, be it "smart" or otherwise. Here too you must pay your carrier for text-messaging services, and while it's ad free, longer missives are sliced and diced to meet the 120 character-per-message max. You also have to "white list" your friends, because flipMail only allows messages from pre-approved email addresses to get through.