Open source (FLOSS) subtitle editor
I recently wanted to merge 2 subtitles files so as to adapt them to a video file that was a merge of 2. Which means I had to basically postpone all subtitles of the second part by the duration of the first part. I eventually found Subtitle Edit, which is open source and seems very feature-complete even though I only used a couple.
It seems to be well maintained, version 3 has been around since 2010 and the latest update was only a few weeks ago. It notably offer tools for automated error detection/fixing (like overlapping texts), can automatically merge short lines, split long lines, renumber lines (useful in my case since I was merging two files numbered starting at 1)… It makes synchronization really easy, for instance by setting a first point then a second point, it will adjust position and speed so that everything matches (and if speed is already good, you just need to set one point and it will adjust position only). I think now that I’ve found this I have a few subs that could benefit from that sync 😉
Last but not least, it also has a spellchecker and can auto translate via *cough* Google.