7zip is open source, but effectively Windows only. Other apps claim to read the same file format on other platforms, but I've not tried them.

Mostly I didn't feel there was anything wrong with TrueCrypt, I just didn't need its extra features. 7zip felt lighter weight. It doesn't need a device driver or need to mount a disk. It also does compression and makes files as big as they need to be, and TrueCrypt seems geared towards fixed-sized volumes which, without compression, tend to be very large. I tried using the Windows O/S compression within a TrueCrypt volume, but it wasn't enough. I could have used 7zip for compression and TrueCrypt for encryption, but that would be more complex.

One benefit of TrueCrypt is the plausible deniability. I don't need that. Another is that it encrypts a whole volume, including things like temp files, which is good for security but again not something I need because my files are in clear on the disk anyway. I am just encrypting backups so I can store them in DropBox.