Just had a look at the stats on this blog and I noticed “straight line” among the search engine terms.
Then I realised that there is no straight line tool in GIMP, which is strange because I use straight lines all the time. Personally I often use multiple short straight lines and a soft brush to approximate curved edges when painting a Layer Mask.
Strangely I have no idea when or how I learned to do this, must have discovered by accident at some stage. It’s is however very simple, in Windows at least, then again everything is once you know .
1. Select the “Paintbrush” or other tool.
2. Left mouse-click where you want your line to start.
3. Press and Hold the “Shift” key on your keyboard.
4. Left mouse-click where you want your line to end.
Viola, you should now have a nice straight line.
I just checked and this “Shift” key trick works with all the tools listed in the image below for GIMP 2.6.2. I was surprised to see that it even works with the “Healing tool”, once you have first defined a clone point.