And fifth TrueCrypt
Have used Cryptainer LE in the past (free light edition), but switched to TrueCrypt later on; no need for me to look back. Same kind of software, but Cryptainer for one is limited to 25 MB volumes in the free edition.
I also have an external hard drive that I use for backup purposes of my business data. However, since when the time comes for me to actually use the backup my original PC will likely be R.I.P.'ed, so I opted not to format the whole drive but made a file container as large as the drive itself minus the size needed for the TrueCrypt installer and runtime version. This way I can easily access my data on other machines as well. Maybe in hindsight I could have also chosen to set up 2 partitions for this, but hey, this works well enough for me.