Free software, good and bad

I have found a number of pieces of free software that I think are really great, and which I use regularly. Apart from that I really like the idea of using software for free, and giving to the community if you have free time. However, if you have tried to tap into useful software, you may have suffered quite a bit before finding some that are useful to you. Like for pay software, you can have problems with finding ones approprate to your operating system, versions of the operating system you use/it can work on, abandonment, programming bugs, etc.

So I'm going to remark on those that have worked for me and those that have not worked for me. If anyone else wants to chime in, so much the more useful. Also, I'm going to leave it open and include apps, languages and APIs.