TrueCrypt is the only encryption software I use - free, reliable and fast. dm-crypt LUKS is an alternative, and support is built into most Linux kernels, but there is has a very steep learning curve for end users and this has kept me from trying it. eCryptfs and EncFS are both slow.
TrueCrypt is an easy choice.
PS. If your external hard drive is formatted in fat32, read the TrueCrypt documentation carefully and pick the correct options because your max file size is limited to 4GB.
Desktop: Dell, Intel Core 2 Duo E7500 @ 2.93Ghz, 2GiB RAM, Linux Mint 13 Maya.
Net-book: SAMSUNG N148-Plus, 2x Intel Atom CPU N450 @1.66Ghz, 1GB RAM, lubuntu 12.04.