I use this one with my Dell laptop and it works great! it doesn't charge the battery but I don't really need that, just keeps it running during my flight (usually the battery was already fully charged in the AC).

The good thing is that it is very slim and light, so I just carry it everywhere in my laptop bag (good for the car!). Just make sure you buy one with the right tip for your laptop, and if you are budget-constrained, you can always get it on ebay for less than 15 bucks!

IMHO this is much more convenient than a bulky inverter, which only converts back and forth from AC to DC, and lose power/heat in the process. Of course, if you need to power/charge other gadgets such as cell phone, ipod, etc, then the inverter would be a good idea for you. My advise: if you only need it for the laptop, just get a DC adapter similar to the above.