USENIX offers one general technical conference per year, and many specialty conferences on topics such as system administration and security. They have a limited number of scholarships available, along with student stipends that allow some students to be reimbursed for attending USENIX conferences. These funds appear to be given out quarterly according to a schedule.
The USENIX web site is www.usenix.org. Here are some of the things they have going on:
Open Source Software Engineering Competition
Undergraduate Software Projects, Scholarships, and Research Grants