One could argue that these are really adapters, not converters. Because the signals used by DVI-D and HDMI are identical, and only the connector is different, it is possible to make a simple "adapter" to convert from one connector type to the other.
Note that there are two potential problems with this conversion: The DVI signal has no audio associated with it so when converting from DVI to HDMI, the HDMI will have no audio.
The DVI must be DVI-D (digital), not DVI-I (analog). This isn't usually a problem since most DVI is the digital version.
There is no video signal quality loss when converting to/from DVI and HDMI.