High-redshift quasars are often lensed into multiple images by galaxies, as is shown in this image (from Castles). They can be used for accurate measurements of various global cosmological parameters, provided that the mass distribution of the gravitational lens system is well constrained. Since the light travel time along the path to the various images is different, observing delayed versions of the same source event in its various images provides a direct way to measure the Hubble constant, independent of local calibrators. The cleanest systems to do these are in the four-image systems as above. However, in many of these cases, it is obvious that the lensing effect is not due to a single galaxy, but contributions from the group or cluster it belongs to are significant.
Finding groups from external shear in multiply-imaged quasars: We are carrying out a survey of x-ray detections of clusters and groups of galaxies that are required by gravitational lens models of multiply-imaged quasars but are not seen by optical means. Our analysis of the system B1422+231 (the image on the right above) was recently published, and mentioned in the September 2003 issue of Natural History magazine. 
Accurate measures of Time delays between multiple images of quasars: watch this space [4-7]
Prospects of observing lensed supernovae: Won't that be nice? 
Weak lensing: My PhD student Tarun Deep Saini and I wrote a smart paper , but nobody seems to have read it.
My Publications in this field
Full List of publications