I recently was watching the 90's X-men cartoon series and the episode title Days of the Future Past and it got me thinking about how many times has Marvel used this concept? In the cartoon Bishop is sent back to stop the assassination of Senator Kelly by Mystique disguised as Gambit. As a child in the early 90's read an X-men story(non-comic) with the same title that sent Kitty to the past to prevent some similar tragedy. Are their other instances of this story line? and who was sent into the past?
Either "once" or "a heck of a lot", depending on how you count.
Days of Future Past was an incredibly popular and successful comic. (The cartoon you remember was an adaptation of the comic story, with minor tweaks to match the cartoon's team lineup.) It sounds like the version you read was the republished collection of that story.
Because it was such a memorable landmark, several writers revisited the idea of that future. Bishop's future has also been explored in particular detail in the recent run of "X-Factor". (Layla Miller sort of counts as this as well - she sent her knowledge of Bishop's future back in time to her younger self.)
The concept has been reused a few times with different possible futures. However, it's not often been handled by sending a 'messenger' back in time; sometimes other plot devices are used. So: there are a lot of examples of this kind of alternate-future plotline, but few following the exact same pattern (which would be pretty formulaic writing, after all).
Other examples: Grant Morrison did something similar with Sublime's evil-Beast future, showing us the consequences of a single possible decision - Cyclop's failure to move past Jean's death.
In Straczynski's Spider-man anniversary story (issues 498-500), he witnesses and then prevents a future destruction of the world by Dormammu.
If you count it, Cable's entire presence in the Marvel modern day is this plot. He sends himself back to prevent Apocalypse's future. (Cable's trip with Hope also counts as the exact reverse of this idea - going into the future in order to change the present.) But trying to keep timelines straight in the presence of Cable is a near-impossible exercise.