I just found Bitventory, which seems to be an online wallet service that requires a minimal amount of trust (it's the major design goal, at least).
Are there other services with similar goals or functionality?
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Yes there is such a service which provides exactly what your looking for it is called StrongCoin
True client side security requires 2 party signed transactions which aren't currently implemented in the maineline client/protocol.
With a two party wallet both parties (the online wallet service provider and the owner) would need to sign the transaction with their independent private key in order for it to be accepted by the network. The wallet provider never has access to user's private key and thus can never transfers users funds.
Any solutions that exist now are stopgap at best.
Lumi Wallet is a client-side HD secure wallet for BTC, ETH and a load of ERC20 token. it also has 2FA (strangely, many decent wallets doesn't have this)