Software for enlarging photos

[Makavelli]

I know that you can't take and 800*600 resolution photo and blow it up to poster size because it will look crap. What i am wondering is can someone point me in the direction of software that will take an image file, and somehow change it to a higher resolution suitable for posters etc.

EDIT:
Okay did a quick google and i have downloaded a demo of Blowup which is a photoshop plugin. Can anyone recommend any freeware/standalone applications?

 

[Makavelli]

I have gimp but for some reason assumed that i would need some special piece of software to be able to do it properly.