Interesting that using a plain text file with standard encryption is not mentioned. No "vendor locking" and keeps portability at maximum. Not perfect though: all eggs are in the same basket and a very strong pass phrase required (say, +64 bit entropy) to stand against brute force attacks.
For additional security a physical barrier can be used in form of a codebook, for example. A simple laminated piece of paper which can fit into wallet will usually do fine. This will give some leeway if the encrypted file gets compromised.
Simple encrypted text file with entries like this:
email@example.com XYZ restOfThePassword
where XYZ is your codebook lookup key should give a decent protection assuming the codebook entries are strong enough (say, 40 bit entropy).