There are several softwares that start out as freeware, however as the popularity of the softwares grows, they are quickly converted to paid software. So the next time you want to install the software and you do not own a copy of it, you may have to fork out some money to use the software again.
However a Site called Last Free Version will help you download the last free version of the software you are looking to download legally.
The navigation of the site needs a bit of work and there is no search functionality available as of yet, but you can browse the freewares alphabetically and use the browser search to find the software you are looking for.
So the next time you are looking to download a free software that went paid, take a look at Last Free Version to see if they have it available.