Librarians' Internet Index has a nice write-up of the information available from opensecrets.org about the 2008 Presidential Election:
Compilation of data on campaign contributions for the 2008 presidential
election. Candidate profiles include total funds raised and spent, cash on hand, debts, and a breakdown of sources of the funds (such as individual contributions). Also includes week-by-week comparisons, a donor lookup, contributions by industry, and other data. From the Center for Responsive Politics, "a non-partisan, non-profit research group ... that tracks money in politics, and its effect on elections and public policy."