b) is ripple (I already commented about it in this thread) - it really is amazing technology that I hope becomes mainstream. It solves a lot of the major problems of BTC and other Altcoins.
No, the ripple protocol and ledger are decentralized. The aspects which are more "centralized" than bitcoin are the XRP distribution scheme (with OpenCoin/Ripple Labs initially owning 100%), and that ledger consensus relies on a hardcoded list of validator nodes (unique node list aka UNL), rather than proof-of-work.
Redemption of IOUs is still a centralized process - you can only redeem IOUs through the issuing gateway. So the difference on ripple is that the order book for trading the IOU is decentralized, and the IOUs can freely circulate. For example, you can't send mtgox USD to another mtgox user (this used to be possible, but mtgox removed their "mtgox codes" feature). But you can do this on bitstamp, because bitstamp is a ripple gateway that issues USD IOUs.
No, the Ripple ledger is not decentralized in any meaningful way. The ledger consensus is entirely decided on by a cluster of nodes explicitly approved by OpenCoin Inc (and in many cases run by them too). There is no way for any non-approved node to influence their ledger consensus; all non-OpenCoin nodes can do is double-check the ledger agreed on by the OpenCoin nodes.
In practice this means that they can freeze arbitrary coins and accounts and there's nothing anyone can do about it. What's more, unlike in Bitcoin it has protocol-mandated fees that are effectively paid to OpenCoin Inc (they have to be paid using XRP which is then destroyed, and the only source of new XRP is OpenCoin Inc) - and those fees are set as part of the ledger consensus. So if it takes off they can ramp up the fees they charge to use Ripple arbitrarily and no-one can do anything about it.
Anyone can run a validator node, and you have total control over your own UNL (usually published as ripple.txt). It is true that the current network of validator nodes isn't very diverse, but rippled has only been open source a couple of months. That will change as more gateways come online (e.g. the xrptalk validator node has snapswap in its UNL, as you can see https://xrptalk.org/ripple.txt).
OpenCoin/Ripple Labs can't steal funds at any account (unless they have the private keys to that account). They could attempt to freeze an arbitrary account by ignoring any broadcasted transactions associated with that account, and only voting on candidate sets which don't include them. But that would be noticeable, as the transactions would get stuck in the candidate sets of all the honest nodes. If there are enough honest nodes with overlapping UNL's, then the Ripple Labs nodes would have to accept it, or be split/forked from the network.
The paid transaction fees are destroyed, so they are effectively paid to all XRP holders, in proportion to their holdings. This is actually similar to the proof-of-stake scheme in ppcoin.
Also, the base fee gets changed by upgrading rippled. There is a pseudo-transaction associated with changing the base fee and the minimum reserve, but these pseudo-transactions are used to prevent the network from agreeing to a fee change (or implementation of new features) until enough validator nodes have the upgraded ripple (upgraded nodes will "vote" on the fee change).
There is a fixed/finite amount of XRP, just like there is a fixed amount of BTC.
The Ripple protocol destroys negligibly small amounts of XRP with each transaction as an "anti-transaction SPAM" measure. It's a negligible amount of money. This anti-SPAM fee currently equates to about $0.0000001 per transaction. If you hypothetically applied that fee to every single bitcoin transaction that has ever occurred, it would total a whopping $2.64. [1]
It's pretty misleading to suggest that fees are "effectively paid to" Ripple Labs (formerly Opencoin) as if it's some greedy scheme. It's actually quite an elegant design to protect the Ripple network from DDOS attacks.
Yes, this does very incrementally benefit ALL holders of XRP, since it has the effect of decreasing supply. And yes, Ripple Labs is the largest holder of XRP.
But no one is laughing their way to the bank collecting some sneaky fees....