FollowTheMoney.org is a website dedicated to informing people about the amount of campaign contributions each politician has received.
OpenSecrets.org is similar, but also gives other information about such things as news, events, and personal wealth and net worth of each candidate or politician.
It is unclear how often the information is updated. The information does not always match what we see in the news articles.
Below are the websites for both. They are interesting to browse.
Update: Maplight.org is another good resource on money in politics.
(Updated)
OK, Fine. 
MapLight is another one.
Great, thanks.
Thanks for sharing these links.