Free open source on-the-fly encryption software
It is important to note that TrueCrypt is disk encryption software, which encrypts only disks, not RAM (memory).
Keep in mind that most programs do not clear the memory area (buffers) in which they store unencrypted (portions of) files they load from a TrueCrypt volume. This means that after you exit such a program, unencrypted data it worked with may remain in memory (RAM) until the computer is turned off (and, according to some researchers, even for some time after the power is turned off*). Also note that if you open a file stored on a TrueCrypt volume, for example, in a text editor and then force dismount on the TrueCrypt volume, then the file will remain unencrypted in the area of memory (RAM) used by (allocated to) the text editor. This also applies to forced auto-dismount.
Inherently, unencrypted master keys have to be stored in RAM too. When a non-system TrueCrypt volume is dismounted, TrueCrypt erases its master keys (stored in RAM). When the computer is cleanly restarted (or cleanly shut down), all non-system TrueCrypt volumes are automatically dismounted and, thus, all master keys stored in RAM are erased by the TrueCrypt driver (except master keys for system partitions/drives — see below). However, when power supply is abruptly interrupted, when the computer is reset (not cleanly restarted), or when the system crashes, TrueCrypt naturally stops running and therefore cannot erase any keys or any other sensitive data. Furthermore, as Microsoft does not provide any appropriate API for handling hibernation and shutdown, master keys used for system encryption cannot be reliably (and are not) erased from RAM when a computer hibernates, is shut down or restarted.†
To summarize, TrueCrypt cannot and does not ensure that RAM contains no sensitive data (e.g. passwords, master keys, or decrypted data). Therefore, after each session in which you work with a TrueCrypt volume or in which an encrypted operating system is running, you must shut down (or, if the hibernation file is encrypted, hibernate) the computer and then leave it powered off for at least several minutes (the longer, the better) before turning it on again. This is required to clear the RAM (see also the section Hibernation File).
* Allegedly, for 1.5–35 seconds under normal operating temperatures (26–44 °C) and up to several hours when the memory modules are cooled (when the computer is running) to very low temperatures (e.g. –50 °C). New types of memory modules allegedly exhibit a much shorter decay time (e.g. 1.5–2.5 seconds) than older types (as of 2008).
† Before a key can be erased from RAM, the corresponding TrueCrypt volume must be dismounted. For non-system volumes, this does not cause any problems. However, as Microsoft currently does not provide any appropriate API for handling the final phase of the system shutdown process, paging files located on encrypted system volumes that are dismounted during the system shutdown process may still contain valid swapped-out memory pages (including portions of Windows system files). This could cause ‘blue screen’ errors. Therefore, to prevent ‘blue screen’ errors, TrueCrypt does not dismount encrypted system volumes and consequently cannot clear the master keys of the system volumes when the system is shut down or restarted.