Once you have good solid text editor like UltraEdit, you want to have another tool to compare and merge files.
SourceGear DiffMerge is the tool I use to visually compare and merge files. It is easy to use and saves a lot of time for me when applying changes to the different versions of source code. And it is free for a single user pretty much any number of users -- as Paul Roub corrected below. I recommend you give it a try.
Sphere: Related Content
Just to be pedantic and annoying, I feel compelled to point out that SourceGear DiffMerge is free for pretty much any number of users.
Glad you’re finding it useful…
Paul Roub, SourceGear