Here are some projects / code I've worked on. This is all free to use, modify, blah blah just try and give me credit, or I'll chemically castrate you.
Here's some JS code that runs an AJAX autocomplete menu. It uses mootools for its AJAX calls (v1.2 required). This is what we use on beeets.com for searching tags.
This code sends an AJAX request to whatever URL is specified during initialization, and puts the response (should normally be a <ul><li> list) into a div right under the input box. Example CSS and HTML is included.
This is a quick wrapper around an S3 class by Donovan Schonknecht that will recursively upload your files in a subdirectory to an S3 bucket, applying Cache Control control headers as it goes.
The original intent was to have a quick way to sync images on a development machine with a live S3 bucket that was serving images on a site, without having to use any crappy third-party software. This was spawned out of the need for a simple uploader that actually works. So far it's been great.