If you have a Windows OS and a good backup of your system (i.e. Macrium free), you can use something like parted magic (see sourceforge.net) to just resize your partitions and problem solved.
The other method usually involves you removing applications from one drive and then reinstalling them on the other. Programs like Internet Explorer, Firefox, and most web browsers for that matter will let you move your cache drive to another location, and that might save you some space, but not much.
I'm on my old Windows XP computer right now, but I remember that Win 7 lets you also resize partitions from within the administrative tools.
There's also an option from Vista-7 through dos called 'mklink /j' that lets you move folders and create a link to them. In Linux, it's the same concept as the ln -s command. (You can probably edit fstab after copying your data to a new folder as well.)
Mac's OS I'm not real sure about, but being Linux-like they probably have something similar.
Sorry if this is "too much info"; you'll have to make some choices depending on what you think is easiest for you.