WaveSurfer A well-designed and user-friendly piece of software from the KTH Centre for Speech Technology, Sweden.
Speech Analyzer SIL International's in-house acoustic analysis package. Fairly easy to use.
SFS (Speech Filing System) packages These packages, authored by Mark Huckvale, are downloadable free from the University College London Phonetics department website. The software available includes RTSpect (a program which allows you to display wide- and narrow-band spectrograms in real time), HearLoss (simulates various types of hearing impairment), and Enhance, which filters various types of noise out of sound files.
Praat This extremely popular package has innumerable features but is oddly designed and far from user-friendly. There are, however, several useful guides to Praat available online:
The Yahoo Praat User Group is the most important online user-group for Praat. Questions are more often than not answered by Paul Boersma, one of Praat’s two authors.
Additonal helpful information is provided by Pauline Welby.
Akustyk Bartek Plichta's add-on to Praat. Akustyk is designed to facilitate accurate and sophisticated acoustic analysis of large data sets. Its functions range from preparing audio files for analysis to producing high quality plots and graphs. Comes with a large number of helpful multimedia tutorials.
Audacity Allows recording, playback and editing of sound files; comes with noise filter and various effects. Releases for Windows, Mac OS X and Linux/UNIX.
STx Real-Time Analyser Free trial version of the Acoustics Research Institute of the Austrian Academy of Sciences' S_TOOLS-STx v.3.7.4 package. Functionality is limited as compared to the full program, but RTAnalyser allows real-time visualisation of spectra (simultaneous FFT and LPC), spectrograms and waterfall plots.