Download managers are very handy tool if you either download a lot of files or download less but larger files from internet. These tools manage your download more efficiently. They speed up the download process by dividing the file into multiple segments and downloading all of them parallel. If your file is hosted on multiple server it will download each segment from different location even. Download managers can also be used to schedule the download or queue them to be downloaded. Just put the download in queue and leave the computer rest will be managed by Download manager utility.
Now you must have used a separate application which are free as well as Paid (some advanced one), but if you are using Firefox, you can use a free Add-on DownThemAll, which can do all these things which any other download manager application does and enjoy the benefit without installing any separate application. The only difference here would be instead of opening the application you need to keep Firefox window Open in order to use DownThemAll. Since it is added as and Add-on to Firefox using it is obviously more convenient and secure. It allows you to scan entire website for available downloads with just one click.
Since it is available as Add-on you can run it on Multiple OS and any platform which is supported by Firefox. If your Firefox running you can install DownThemAll. DownThemAll can also be used to verify downloaded file by entering Checksum Hash in the form at the time of download. All the queued download are displayed in a list manager which also shows Download speed, size of file, progress bar, Percent completed and time left for completion.
With DownThemAll installed A right click on any page will display all downloadable file on that page by selecting option DownThemAll. Once selected it will display list of all files, pictures and media files being used on the page and available for download in a separate Firefox tab. You can also download video embedded on any page which are not downloadable otherwise. You can also create filter and only files which match the filter criteria will be downloaded.
Apart for all these functionality, it also have preferences through which you can configure how DownThemAll will work or behave. Overall DownThemAll is best available download manager which is also free if you are a Firefox user. You can install it from Mozilla Add-on repository and it is also compatible with future version of Firefox that is Firefox 4.0.