That really depends how many dimensions you are looking at. If you are imaging cells in 2D, on a culture plate for example, then you can use the "manual-tracking plugin" for imageJ (http://rsb.info.nih.gov/ij/plugins/track/track.html) to follow the invasion of individual cells and create an invasion index.
If you are using a 3D invasion assay such as Matrigel or other organotypic systems then you could try software such as Imaris.