For some transaction, the possibility of generating "chargebacks" might be very useful. For example, when trust cannot be established between both parties. In such cases, a trusted third party may be very helpful. One very well known and reputable Bitcoin escrow service was ClearCoin - however, that's currently not available.
Please list one service per answer (not multiple services in the same answer) so that the votes can decide on reliability (use comments for testimonies, if there are any ;-) ).