Visual SubSync is an actively-developed and stable subtitling software. I used it for a commission about 18 months ago, and found it to be excellent. It’s freeware, and writes standard .srt files so if you’re distributing on a DVD (such as sending a movie to a festival) then viewers can choose to load subtitles or not.
And YouTube seems to support the upload of .srt sub-title files along with a video, but it’s not something I’ve tried.
To allow SubSync to load up video files, and write subtitle files, you may need to give it Administrator rights under Windows 7.
Usage is a little unintuitive at first try. The basics are: Select the segment of audio waveform that you want to make a subtitle for, by clicking on the start of it, holding down Shift, and then clicking on the end of it. Then right-click on the selected/highlighted portion of audio, and select “Add subtitle”. Now type the words into the lower box and hit return on your keyboard. The text you typed will be placed alongside the timings for the selected/highlighted portion of audio.