There are two software projects that have had huge influence on the quality of free software, and I wanted to mention both in case someone do not yet know them.
The first one is valgrind, a tool to detect and expose errors in the memory handling of programs. It is easy to use, all one need to do is to run 'valgrind program', and it will report any problems on stdout. It is even better if the program include debug information. With debug information, it is able to report the source file name and line number where the problem occurs. It can report things like 'reading past memory block in file X line N, the memory block was allocated in file Y, line M', and 'using uninitialised value in control logic'. This tool has made it trivial to investigate reproducible crash bugs in programs, and have reduced the number of this kind of bugs in free software a lot.
The second one is Coverity which is a source code checker. It is able to process the source of a program and find problems in the logic without running the program. It started out as the Stanford Checker and became well known when it was used to find bugs in the Linux kernel. It is now a commercial tool and the company behind it is running a community service for the free software community, where a lot of free software projects get their source checked for free. Several thousand defects have been found and fixed so far. It can find errors like 'lock L taken in file X line N is never released if exiting in line M', or 'the code in file Y lines O to P can never be executed'. The projects included in the community service project have managed to get rid of a lot of reliability problems thanks to Coverity.
I believe tools like this, that are able to automatically find errors in the source, are vital to improve the quality of software and make sure we can get rid of the crashing and failing software we are surrounded by today.