FTW Transcriber from Tyger Valley is free transcription software. It seems to have a couple of features that Express Scribe doesn't have, such as playing files on your local machine or server instantly without having to load into the program. It doesn't have the noise filtering features that Express Scribe offers. The interface is simple to use, with buttons similar to a tape recorder or VCR. It can also play videos and it supports the three largest brands of foot pedals.
One of the reasons the software is free is because it's ad supported while Express Scribe is not. Ads play at the bottom of the screen. If that doesn't bother you, it may be a good alternative to Express Scribe.
FTW Transcriber should work on all PC machines with Windows XP and later. The FAQ states that the program "has not been tested on a MAC."