http://en.wikipedia.org/wiki/United_States_Republican_presidential_candidates%2C_2008#Other_candidates for the Republicans. 
 http://en.wikipedia.org/wiki/United_States_Democratic_presidential_candidates%2C_2008#Other_candidates for the Democrats. 
 There are apparently a lot of generally unknown people who somehow qualify for the ballot in some state or another.  Most of them are apparently so little known that they don't have Wikipedia articles about them.  Fewer than five of them actually do.  Neither Gravel nor Keyes fits into this "other candidates" category.  Both are significant, known people who are on the ballot in many states.