Lately I've been using the free version of Voyager navigational planning software, from Seattle Avionics. It's impressive. You tell it the parameters of your airplane, although unless you fly some strange rare airplane, or it is customized like crazy, you'll probably find it already in the list, and you can edit list entries to match your aircraft.
You tell it your point of origin and destination, whether you're flying IFR or VFR and what altitude you'd like to fly. It will suggest corrections for wrong-way altitudes and plot a route for you. Meanwhile, if you have an internet connection, it downloads TFRs and weather so it can show you the conditions you're going to encounter. The profile view shows control zones and forecast ceilings, and my favourite part is that it interpolates all the available upper wind data and calculates a trip time. It's a breeze to try different altitudes (I think the pay version does this for you) and see where I get the least headwind component over the trip, and the times it comes up with have been to-the-minute accurate.
It does far more than this, but I use company software for weight and balance, and file my flight plans by telephone, because I have to check NOTAMs anyway. Software like this feels like cheating. I imagine flight students do use things like this to cheat on their homework assignments. (I say they are cheating because they aren't allowed to use software on the exam, and if they've always used it in practice, they're likely to make errors on the exam).
If I'm using it just to give an estimate to a customer I don't bother entering an altitude, so it just uses the one from the previous flight. This generates lovely precise warnings like Extreme danger! Crash into terrain expected at 475.4 nm (2:56) into flight. I'll also use it to see how many hours it will take to get from Florida to Washington State, for example, and just enter the endpoints, in which case it gravely warns me Insufficient fuel! Fuel will be exhausted before the destination is reached! It will still crank out the numbers though, which is all I want, to see how many fuel stops and how much time-before-maintenance I'm going to need.
Seriously, the most difficult part of flight planning with this software is deciding where to stop for fuel, and I understand that the pay version will even do that for me. As it is, I just guess a likely airport en route and designate it a "stop-and-go" waypoint, with refuelling, and the flight plan is adjusted for me. My weakness is that the outlines of the states are shown, but not the names, so I look at a vaguely rectangular shape in the middle of the country and wonder "Is that Oklahoma or Nebraska?" Remember please, before condemning me for my geographical ignorance that this is not my native country. I can draw a fairly accurate map of Canada showing all the provinces and territories and name and site their capital cities. I'm just a little vague on whether I'm overflying Wyoming or Colorado at any given moment. I always think Wyoming should have more cows, actually. I read a romance novel once set on a Wyoming ranch. I never see any cows, nor romantically entangled cowboys in Wyoming.
Of course, SOFTWARE AND HARDWARE IS NOT INTENDED FOR USE IN THE OPERATION OF NUCLEAR FACILITIES, AIRCRAFT NAVIGATION OR COMMUNICATION SYSTEMS, AIR TRAFFIC CONTROL SYSTEMS, LIFE SUPPORT MACHINES OR OTHER EQUIPMENT IN WHICH THE FAILURE OF THE APPLICATION SOFTWARE COULD LEAD TO DEATH, PERSONAL INJURY, OR SEVERE PHYSICAL OR ENVIRONMENTAL DAMAGE. It's worth noting that the large full-colour weather map on the back of every USA Today newspaper bears no such warning, depicts major fronts, and shows the names of enough cites that I can identify the states. USA Today is actually remarkably useful for navigational planning. For example when your customer asks in the middle of breakfast, "So how long would it take to get to Virginia from here?" Just flip over the free hotel newspaper and use your spread thumb and finger to measure the distance and compare it to the distance of a recent trip for which you remember the time. This makes you sound really knowledgeable if you're talking to the customer by phone, and makes you look like a complete crackpot if the customer is at breakfast with you. Plus the ink doesn't come off on your hands. Seriously, they use some kind of special ink. It used to be the main point on which they advertised the newspaper.