Are there any major current UI wallets (desktop/mobile/web) that support creating / spending multisig transactions?
Armory is a client for advanced users that has support for multisignature https://bitcoinarmory.com/
The DarkWallet wiki states that DarkWallet supports "creating and spending multisig funds". However, the text there further specifies that "at the moment spending and communicating the funds is quite manual, transactions have to be copy pasted from one user to another".
An answer on How can I create a multi signature 2-of-3 transaction?, features code to generate and spend 2-of-3 transactions presumably with Bitcoin-Core, as Gavin created the script.