BitcoinJ: Very old client from back in the day
Still has a team maintaining & updating it (driven by Andreas Schildbach's work on Bitcoin Wallet on the Android Operating system), but it shouldn't be on this list, because:
- Cannot be used for a full validating node (validates block headers only, then resolves transaction validity using BIP37 bloom filters)
- Is only a library; does not function as a standalone, executable Bitcoin client