give another user ability to download without making them an administrator

Depends on what you want them to download. If you want to let them download and install software, that may very well require an admin account (depending on the software). For other downloads, no admin account is necessary.

However, note that if you are giving this roommate unsupervised access to the laptop, he/she can get access to whatever is desired, even without the password for your account. There are tricks that a knowledgeable user could use to get access to everything in your account, as long as physical access is given. If that is a concern, you should consider activating FileVault encryption as well as setting a firmware password. (How the latter is done depends on what version of Mac OS X you are using.)

To clarify one point, if you give another person administrator privileges, he or she has full access to all data on the computer. Whether FileVault is on or not makes not the slightest difference.

No special privileges are needed merely to download files. Any user can do that.

how do i allow another user to sign in from cold start-up.....my log in is the only one that comes up and as the adminsrtator it requires my password.