In some places, where the airport is far from town, airport rental car rates can be surprisingly high and there are few if any off-airport branches nearby. (Ex Kansas City, Norfolk, Cedar Rapids) This makes shared shuttle vans a competitive option. In fact, I've used Super Shuttle in Kansas City. They have a presence in several markets. The rates are relatively good, especially if the group you're visiting has a corporate rate plan. The downside is they frequently circle around for an hour and leave once the blue 15 passenger van is finally full. (One row of seats is removed to make room for luggage, so it's not necessarily as annoying as you might think.)
Hotel courtesy shuttles can also be a good deal. I've used many of them, but be advised that those hotels tend to be in heavily travelled areas where the rates can be high. Also, with all shuttles, watch out for reliability and scheduling glitches.