...that can open multiple text files at the same time and highlight any difference between them. Any ideas?

Beyond Compare 3 by Scooter Software. It works on multiple platforms, does text, binary, or CRC compares, will compare all contents of a directory or any archive file, will even let you drill down into sub archives as deep as you want and compare individual bits. The contents compare pane is the best I've ever used.

It's got a free trial. It's totally worth the 50 bucks for a full license if this is something you frequently have to do.

30 days of trial... thanks, but I was hoping for something freeware. I'm just testing a program for someone and this would help me identify and fix the odd syntax error in the output files. It's not something that happens often (or that I have to do in the first place) but even if it were, I'm in no position to fork out 50$ just for that.

Something in the 'diff' family perhaps?

http://en.wikipedia.org/wiki/Diff

I've used Diffuse to compare multiple files.

WinMerge is also good, though I'm unsure if it handles more than a two-way compare.