At this point, midday on Christmas Eve, basically all shipping deadlines have passed. If you're lucky you can find a store that has items for people on your gift list, but let's get real: You're choosing between gas station beer cozies, dancing reindeer and clearance VHS tapes. If you want to find something that will please your peeps now, I have a few ideas for you below.
* Paid upgrades. A lot of people of varying levels of geekitude are users of "freemium" services like Flickr and Evernote. They'd probably appreciate an upgrade to a paid account (or an extension of their existing paid account). This is nice because it's practically instant, and it isn't like the size or the color will be wrong. Print the email out and put it in a card, or get super creative with the presentation, and you're done.
* Online smarts. You can get a gift account for something like Lynda.com or Safari and help your nerd friend achieve +5 Intelligence in the subject of their choosing. There are a wide range of topics here, so your recipient will likely find something they enjoy.
* Software licenses. Know someone who uses the previous version of a particular application because they never got around to getting that new license? You can get a lot of software licenses online now, so buy someone you love a license for that program they love. Or encourage someone to dive into software you already love: Something like Scrivener (if they are writer-types) or TextMate (if they have been really good).
* Geek Time. If you are a consultant by trade, or just the alpha geek in your particular circle, get a card for someone and offer them a block of time with unlimited access to your brain. Or give them a certificate for a specific task: help setting up that iPod, upgrading to Snow Leopard, or implementing a backup solution. (Tip: This can be a team gift, if you know someone else got them some tech.) There are a lot of ways you can go with this one.
* Photo albums online. It always seems like someone is missing at Christmas time, so what better way to include them? Get some nice shots of all the gift opening and toss them on the photo sharing service of your choice. Get a short URL from someplace like bit.ly and customize it, then send it off in an email or give the person a call. Get extra points for having a group photo holding up a "Merry Christmas Uncle Albert!" sign.
If you have any additional ideas for last minute gifts, let's see them in the comments.
Update #1: ryemac3 has a great suggestion: vanity domain names. Get your parents the site they've always wanted at 'thewilkersons.net' and watch the fun!