Though things are beginning to change, Denver is first and foremost a car dependent kind of town. 

However, if you are here on business and don't need or want to venture beyond downtown or lower downtown (LoDo), then you don't need a car and you shouldn't need a taxi (unless your ability to walk 3 or 4 blocks becomes affected).  The16th Street Mall shuttle is a free bus that runs up and down 16th Street between Union Station and Civic Center and makes getting around downtown and LoDo very easy and convenient.  Should you decide to visit a restaurant or site just beyond downtown, then use a cab.  Around Denver, the best way to get one is to call one of two easy to remember numbers: Number 1 and Number 2.   

If you come to Denver and stay out on Quebec because hotels are less expensive there, then more than likely  you will want a car.  Quebec is out by the old Stapleton Airport, and it is not convenient to downtown or any of the major sites.  Other than a rental car, the only economical transportation option between the Stapleton area and the interesting stuff is the bus, which limits your flexibility and wastes your precious vacation time, as a taxi would be unreasonably expensive. Only if you are on the tightest of budgets would I suggest using the bus system.  Those who live where public transportation is abundant and never use cars might find the public transportation system in Denver intolerable due to its limited schedule and routing. 

If you come to Denver and plan to do some exploring in the nearby mountains, then there is absolutely no choice.   You need to rent a car. 

While the most interesting parts of town are within a relatively small area, driving is, unfortunately, still the most efficient way to get around.   Luckily, by big city standards, Denver is a fairly mellow town, and the driving is not too bad (except on I-25).  And, compared to other big cities, parking is not yet too much of a problem here.  So, while that stays true, a car is a good choice for the increased flexibility.   Taxis are expensive and the bus system is too limited to be very practical  (or tolerable) for those with a choice.  Also keep in mind that because Denver is such a car town, taxis are generally not available if you don't call for one.  Then you have to wait for it to arrive. 

Parking at any of the downtown hotels will run close to $20 a day.  That's expensive, but there are alternatives.  You could rent a car, then find a parking lot/garage that allows overnight parking for less than what your hotel would charge.  See this website for more info on overnight parking in downtown Denver   If there is a sporting event while you are in town (and that is likely), parking rates at non-hotel lots may spike.  So, keep an eye on what's happening over at Coors Field, in particular. 

Parking meters cost $1 an hour in Denver.  Be aware that the meters in LoDo (lower downtown) operate until 10 PM every night except for Sunday.  In Cherry Creek and around Larimer Street, if you don't park in a lot, a garage or at a meter, be sure and look out for the pay stations on the sidewalk.  It may look like you've found a free (meter-less) spot, but you most likely need to pay for some time at the pay station and then put the ticket on your dashboard. 

Use the following map of downtown parking options to help you out.  It's a great resource.