If you work at the ssh command prompt all day long (like I do), and if you need to compare text files and merge differences (like I do), then make sure you check out vimdiff (thanks to Jeff Roberts for bringing it to my attention).
If you run 'vimdiff file1 file2', the tool will split your screen vertically, with file1 displayed in a vim session on the left and file2 on the right. The differences between the 2 files will be highlighted. To jump from difference to difference, use ]c (forward) and [c (backward). When the cursor is on a difference block, use :diffget or do to merge the difference from the other file into the file where the cursor is; use :diffput or dp to merge the other way. To jump from one file's window to the other, use Ctrl-w-w. Google vimdiff for other tips and tricks. Definitely a good tool to have in your arsenal.
If you have the luxury of a graphical enviroment, I also recommend meld (thanks to Chris Nutting for the tip).