There are plenty of online wallets (e.g. Ethereumwallet, MyEtherWallet, Jaxx Bitcoin and Ethereum Wallet - Chrome extensions, etc.) but I did not find any wallet that has the ability to add a contract address to or to allow us to interact in any other way with smart-contracts. Do you have more data?
Not at this time. You can use Jaxx and MyEtherWallet to send transactions with data and increased gas, which means you can send ETH to a contract. But fully interact with a custom ABI does not appear on any light / web wallets at this point.
We (MyEtherWallet) are currently working on it. We've done one-offs for Digix and Augur and just implemented the Digix claim functionality (will be live after testing on April 24th, 2016).
Hopefully we'll launch the ability to interact with smart contracts in ~2 weeks?
I recommend getting your Mist set up and running anyways. They have more resources and time that we do and will most likely always be a couple steps ahead of us. :)