I am a big fan of TinyUrl. It is a very simple concept but one that has so many applications. If you have ever seen a long URL wrap around and therefore break in an email client then you have one of the use cases for using TinyUrl.
Another case (which I imagine is converging as mail readers get more and more mature), is to make sure that the widest audience can see the link. Now that we live in a world of RSS and syndication every where, you have no idea when URLs that contain "border line" characters are going to get truncated or cause parsing errors in one of the many processors that it must go through in it's journey to the millions of consumers it is serving.
Now that I am using twitter often and blogging more often I find myself having to go through the following steps:
highlight the link I want to copy and hit ctrl-c
hit ctrl-t to open a new tab
type in "tinyurl" and hit ctrl-enter
hit ctrl-p to paste the link and hit enter
select the link and hit ctrl-c
paste it to where I want it to live
This was getting old to me... so I just googled "tinurl plugin" and found this wonderful little gadget, the TinyUrl Creator :: Firefox Add-on.
So now my 6 step process above is a right click, left click, ctrl-p. As Borat would say, "Very Niice!"