Avoid TestKing. It's a braindump, and therefore MS considers it cheating.
I'm not familiar with SelfTest, but I have heard good things about TrainSignal and have had great success with TestOut personally.
If all you're looking for is a practice test then MS pushes MeasureUp pretty heavily too.
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CompTIA A+, Network+, Server+, Security+
MCTS:Windows 7
MCTS:Hyper-V
MCTS:System Center Virtual Machine Manager
MCSE:Security 2003
MCITP:Enterprise Administrator