There are quite a few occasions in digital life where you want to convert that 'dot something' to 'dot something else'. And unfortunately there is no single solution to do all of that. Nevertheless, with a few service oriented websites around, we do have a few options.
I can write two pages on how much effort I spent eight years back to convert a .bmp file to .jpg or .gif. All because of meaningless standards (well they make sense now) then. We had MsPaint, different in each version of windows, which doesn't open or save certain files. Then here comes Windows XP and all of your earlier tips and tricks are no longer required. Now, it comes with its own limitations and the next 2 years, it is useless time spent on figuring the same and more things for XP. Tweak it, modify it and crib about it. And now, here comes Vista now :-)). You see this is how the 'Software Development Lifecycle' is. Software just keeps versioning itself and it never ends and that's exactly why it is called a Cycle.
While the above websites convert Multiple file formats, given below are a few that only convert videos.
The most frequent need to convert files arises from dealing with photos (pictures). So, here is a list of places where you can edit or resize your pictures 'online'. There is no need of any software to be installed and this is the perfect thing to have, if you need to edit/shrink pictures instantly, even at office. Check out the best websites for this in the Photo Edit section of this website.