*First*: Check Pioneer Careers Online for available positions. This website gives you inside track information about positions at DU, around Denver, organizations associated with alums, etc.
*Second*: LinkedIn is an excellent job search and networking resource for any industry. Even without pursuing an internship, you should be creating and utilizing a professional LinkedIn profile by the end of your undergraduate studies.
Idealist.org and Colorado Nonprofit Association are great for jobs in the nonprofit sector, Idealist includes international listings. Campusaccess.com provides contact information for major agencies and NGOs. VolunteerMatch.org lists both volunteer and internship positions with non-profits, nationwide.
LookSharp, InternJobs.com, and Internshipsabroad.com are all useful internship search sites. Internship Search Engine, Summer Jobs, and traditional job search sites like Monster.com, CareerBuilder.com, and Craigslist can also be used for internship searches, especially in the private sector.
USAJobs is the one-stop-shop for internships and career beginnings with the US Federal government's many agencies. Some agencies, such as the US Department of State, have further information on their websites.
WashingtonPost.com has many job listings as well as a terrific "How To" Section covering salary negotiation, resumes, federal careers, and interviewing. College Grad is another great career advice source for recent graduates.
GuideStar is a great directory of nonprofit organizations.