WARNING: Using TrueCrypt is not secure as it may contain unfixed security issues.
homepage
http://truecrypt.sourceforge.net/
makeuseof
http://www.makeuseof.com/tag/encrypted-folders-truecrypt-7/
boxcryptor (java based)
https://www.boxcryptor.com/
https://www.boxcryptor.com/en/boxcryptor-portable-download
AxCrypt with Source Code
http://www.axantum.com/
http://www.axantum.com/AxCrypt/SourceCode.aspx
DiskCryptor with Source Code
http://diskcryptor.net/
Freeware softwares based on TrueCrypt source code :
https://veracrypt.codeplex.com
https://ciphershed.org/
VeraCrypt is compatible the TrueCrypt containers.
//csharp project
http://www.codeproject.com/Articles/20880/Folder-protection-for-Windows-using-Csharp-and-con