A slightly modified version of Gentium, named Ungkam: version 1.2 now available in a Mac Suitcase version as well as a Win/Linux OpenType version. Ungkam may be the first calque from Latin into Sumerian.
An extended version of Bitstream Vera, named Lynn: version 0.4 is now available as a Mac Suitcase or in a Win/Linux OpenType version.
An eclectic cuneiform font created for the Unicode standard: available here.
Aegean is a free font created by George Douros which supports Old Persian, Ugaritic, and other scripts relevant to ancient Near Eastern research. You can download a locally cached copy of the distribution here, or go to its home page here.
If you are using Windows XP, you will benefit from enabling ClearType (which is enabled by default on Vista). Instructions for doing this are available from this MS page.