Here are a few helpful sites for anyone looking into US presidents in their family tree. The first one has a great deal of information, but the others I have listed can offer some other interesting information as well. Do you have any ancestors who spent time in the White House?
Genealogy of the US Presidents - Probably the best and more comprehensive resource for anyone researching the family trees of the American presidents. Each president is listed, with their pedigree charts. Some are fairly sparse, but the better known presidents include many generations of ancestry.
The Political Graveyard - Not just presidents, but a huge database of the burial locations of all sorts of US political figures (both well-known and obscure). A fun resource for lovers of cemetery research.
Ancestry of George W. Bush - Since Bush is the current president, I thought I'd give him particular notice. Here is an ahnentafel chart of several of his ancestors.
Dynamic Family Tree of the American Presidents - Similar Gedcom information as the first site listed here, but presented in a very eye-friendly dynamic pedigree chart format. Again, some presidents have little ancestry, but some go back for generations.
I found a neat little blurb about presidential genealogy on Dana Huff's genealogy blog. I couldn't find the original Wikipedia article, so I am linking to her blog entry. Apparently, almost any American family tree with more than 10 generations of history will likely find that they are related to more than a dozen US presidents.