FDM, Jdownloader, Flashget, Orbit and Downthemall(firefox addon) are all very good alternatives. I assume you are finding alternatives because its not allowed in Islam to hack software and not to pay developer for their ownership(I appreciate that a lot). But the things to keep in mind while using alternatives is that they are not the same as paid softwares. Most of them are not as beautiful as paid ones though their productivity is often similar to the paid ones. DO NOT GO FOR THE LOOKS! At first sight, you may or may not like the user interface of the software but you will surely like it after a while.
Second thing is, I suggest you to go to the help section of the free software and learn about it. It will only take an hour at most but will benifit you a lot. No matter how big software freak you are, you always learn something from that section and it really makes a difference.
Thirdly, Don't get disappointed if you do not get along with one alternative. there are always softwares which are free and are better than paid ones. Try another and another and another and you will get something worth your work. And when you do, please share your experience with that software with us all
And lastly, as you are going to use it for video grabbing, video grabbing is not allowed from every site. Some may allow it, some don't. Youtube doesn't. No one is going to arrest you or something and you can make a thousand excuses like "I don't have internet access all the time and need to watch those videos OR They can stop it but they are not which means restriction is just on the papers" there can be a thousand or more but they do not change the reality that it is their website and they do not allow you to download videos from there. But if you look deep, yoou will get something even better In Shaa Allah