I've just committed the first version of a HD wallet generator.
It can generate mainnet and testnet addresses offline.
I suggest that you download and compile the code yourself before running it on a machine that is not connected to a network.
Write down the passphrase carefully because if you lose it, you'll lose all access to the addresses contained within. Similarly, anyone with your passphrase can access all of your addresses.
Some testing has been done but please use caution!
If you find it useful, I'll continue to add features. There is no support for Z addresses - I haven't checked what's involved in Z address generation yet.
Edit: I've added vanity address generation. After generating a passphrase, use zretrieve to search for an address that matches a regex. eg to search for an address starting with "t1jl":
zretrieve -passphrase="board start difference answer blossom roll powerful million rough butterfly bedroom beam" -match "^t1jl" -i