I like to keep up with the development of various types of hybrid vehicles through sites like autobloggreen. A common observation is that hybrid powertrains will give the biggest benefit to vehicles which stop and accelerate frequently. UPS/FedEx trucks, garbage trucks, and city buses are being developed as prototype heavy hybrid vehicles.
I think there is a group of vehicles where the benefits of a hybrid powertrain should be even greater, though: vehicles which run their engines to power on-board systems while they are stationary. I'm thinking of bucket trucks, various lifts, and a lot of hydraulic construction vehicles. There is an office building under construction near mine at work, and I see (and hear) a lot of equipment with the engine running while it holds a load or a person works on a platform. Autobloggreen recently reported on delivery of a hybrid propane delivery truck which can run the delivery pump from the batteries, eliminating the need to leave the truck running during the transfer.
In many of these vehicles, the accessory system doesn't even need power all the time. For instance, the operator of a bucket truck may not need to move the bucket for minutes, once it is positioned. But, currently the engine runs to keep the hydraulic pump ready. By contrast, batteries don't need to use power just to be at the ready. Note that the batteries don't need to be able to power the on-board systems for very long - the engine can still run whenever the batteries need to be recharged, but it doesn't need to run continuously.
There are many more vehicles which are used in this way. If you are alert to it, you will find work vehicles with running engines frequently. Police are another example. I understand that there are enough computers and radios in a police cruiser that the engine needs to run to power them all while the officer works with them (plus, they want the air conditioning to stay on).
I suspect that the construction and utility industries provide a noticeable portion of our fuel use, and although some equipment will have continuous high power requirements, reducing what we can would be worthwhile.