As a member of a very liberal city in a very liberal state, I have to bogart other people's house races for fun.  Two come to mind that I've been watching: Nevada's Second and California's Fourth Districts.  In Nevada's Second District, we have fourth-generation Nevadan Jill Derby, who has a master's and Ph.D. in cultural anthropology from our very own UC Davis and who got the Comstock Cowboys to record her a pretty awesome song, running against incumbent Republican Dean Heller.  Apparently, she's just passed him in fundraising for the third quarter, but she's in a very Republican district and needs your help.  I've actually campaigned for her while canvassing for Obama, so now I feel especially invested in her campaign.  Go to www.jillderby.com and contribute.
In California's Fourth (which I've driven through on my way to campaign for [Obama and] Jill Derby!), Charlie Brown is running for the second election cycle in a row, this time not against corrupt incumbent John Doolittle but for his open seat after his not-so-surprise retirement.  His opponent is Tom McClintock, who is crazy and lives several hundred miles away from the district.  Go to his website to contribute.
And finally, one of the few things on the ballot in my area that is actually very competitive, the No on Prop 8 (anti-anti-gay marriage, basically) campaign is being outraised and needs money.  Contribute somewhere on this site.