I did a couple of research projects, which I found both to be incredibly rewarding.
My senior thesis comprised of looking at a glioblastoma (brain cancer) cell line and exposing it to a protein kinase C inhibitor (staurosporine) to discover how this affects progression throught the cell cycle. I found a previously uncharacterized S-phase delay, and am hoping to publish these results when my research advisor returns from the Czech Republic doing a volunteer semester.
I also did an internship for a summer and worked on moving a transposable element from an intron into an exon of a gene dealing with muscle morphogenesis in drosophila. This also meant staining embryos to look at the phenotypic effects of moving this "piggyback" element.
I have not been published in either project, but hope to be in the first, like I said above. Not meaning to be a thread hijacker, but I love hearing about what others have done. To the OP, were you asking because you have also done undergraduate research. If so, what did you do?