Here are some ideas for projects I've had recently...
Big/Hard Projects
The UBER Music Trading/Management Application!
An application which allows you to enter various data about yourself such as your body type, the food you eat daily, the goal of using the program (feeling better, losing weight, becoming buff, etc.). It will then be able to predict how healthy you are/feel, let you correct it so that it can learn what effects food has on you, and suggest how you should change your diet for optimal well-being.
A piece of software which analyzes a program (both the source code and its behaviour while it's running) and allows the programmer to see the program's overall structure, as well as potential problem-areas and parts which should be optimized.
Whether you want to get off work or school, the best way to fool anybody is to make your body think it's sick for a short period of time!
An IRC Bot that puts HAL to shame.
Medium Projects
An automatic Python package installer (like apt-get).
An advanced GUI interface to del.icio.us written as a Javascript/XUL extension to Firefox (a full replacement for Firefox's bookmark system).
A tree-based representation of your browsing history, showing which pages led to which, and how deep you are from where you started. Also allows you to rearrange and group things.
A peer-to-peer bittorrent searching/sharing network. It'll put suprnova to shame!
A GUI tool for creating advanced google queries quickly.
Neural Network Music Analyzer which will allow visualization systems to respond to very complex sound patterns
A program for managing/browsing/listening-to large MP3 Collections
Collaborative movie rating site with trailers.
A byte-optimized template compiler utilizing enhanced Cheetah syntax.
A web-framework that automatically generates Javascript and DHTML based on user-events. It should make developiong a web-applications more like GUIs, and should result in much less bandwidth and server-load as well as a snappier user-experience!
A dynamic, macro-enabled wiki with server-driven javascript and java/other-gui components.
Small, Fun Projects
A page full of serach fields, like this one that I use all the time, but with a tighter more efficient design, and javascript-assisted navigation.
A program to make CapsLock do something useful (like Spaces-As-Underscores-Lock).
This is a little program that sits in your system tray (or userspace as a daemon, if you're on some WEIRD operating system) and serves files. Why would you need another one of those? Because this one is specifically designed for sharing a single file at a time so you can paste the link to a friend!
An external set of analog dials which can show various statistics about your computer's operation in realtime (i.e. CPU usage, network speed, temperature).
A program for creating funky complex cog formations.
A program which exploits IE's bugginess to install a copy of Mozilla that usurps IE's dominance over the operating system.
A neat idea for a laser projection system (that I came up with while playing with my laser pointer and a beer bottle.)
Hook a computer up to your TV, the easy way!
Other
Possible ideas for my undergrad project.