how bout designing a smart home, office, etc. u may control the house u designed from other computer. control the house means switching lights, open and close the gates, switching fans and etc. it's a very good project plus it does have market value.
There are a lot of variety you can play with microcontroller, from home appliaces, industrial, security/ access control, or maybe more specific market controller.
What the project use for ? Market or just hobby ?
microcontroller and microprocessor have great ability and we can use those to make and invent a lot of things . think about your favourits and what you wanna do .
AND important notice : before make any thing ,choose your IC , which ic do you want to use (AVR PIC or ...) First think about your knowlage and your ability .
i think the best place on net you can find useful project is "http://www.avrprojects.net/"