1) Films - After watching a film or Television program, you might remember a few "trademark" images, for instance, a vehicle or machine. These can become great NXT models with some imagination and practical building skills. Examples of these include, Jungle-Cutter from Indiana Jones, Batpod from "The Dark Knight", "WALL-E" from WALL-E, etc.
2) Machines - Machines are often very good for replicating with NXT, as both their design and functions can be copied using the various sensors, motors, and other elements from LEGO. Examples of these include; Whac-a-mole, Robotized Bulldozer, etc
3) Vehicles - These are probably some of the easiest and most common creations to build, as many require only the basics of NXT building and programming skill to make. Some examples include; Drag Racer, autonomous robots, etc
4) Vehicle/Machine Hybrid - a mixture of the two, these are usually more advanced than a vehicle, and can be very technical, using gears, motors and sensors to get the job done. These are models like bulldozers, mobile cranes, etc.
5) Household Equipment - This group is very similar to the machines classification, but are used for household purposes, e.g. hover, blender, treadmill.
6) Fighting machines - Also branching out of the machines category, fighting machines can be challenging to make and be very entertaining to watch and play around with. Classic examples of these are Sumobots. Have a look at some here
7) Challenges – A challenge such as those on NXTlog can give you an idea for a model you might otherwise have never built. Check NXTLog for some great models from such challenges here, and have a look at previous challenges from NXTLog here
See what else you can think of. There are still many good ideas for building models waiting to be found. Think of what you have, what functions your creation would need, what sensors and other hardware you can/have to use.
And most of all, have fun challenging yourself!!!
More inspiration can be found on the NXTStep.