I have an employee who is always 15-20 minutes late, the reasons vary but she always has an excuse. Any suggestions on how to handle this?

First, is she the only employee who has this "problem?" If not, you had better make sure you are addressing all employees in the same manner. First, talk with her and find out what is going on; is she late becuase of a child care issue (or other legitimate situation) and can you accomodate it by allowing her to work 15 minutes later each day? (You may be setting a precident here so be careful). If she just can't get up in the mornings, begin the written warning process.

If this employee is in a role where she needs to be available durring specific times, this cannot be allowed to continue. Seems to me that the problem could be corrected by starting her daily routine a half hour earlier.
If she does good work and her tardiness is not causing a major disruption, would it be possible to alter her work schedule to start and end a half hour later? This would reduce resentment in other employees who feel she is getting away with something, but it will likely result in others wanting a schedule adjustment as well.
I've also seen employees who have been granted a schedule change this take advantage of it - the 8:15 arrival gradually sildes to 8:45 after the start time is changed from 8:00 to 8:30. Or, they will stay for only 15 minutes after everyone else leaves, instead of a full half hour.
As for the mechanics of handling the issue, use whatever process you have in place - formalize your expectations and make sure the employee sticks to them.

Anonymous? Are you my boss? I have been doing a lot better! Come on!

When people accept employment, they are told what time to report to work. If there is a problem with this time, they should address it with either their hiring officer or their immediate supervisor. I don't care why she is late, she is late and it should be addressed. You have others in your department and this could affect their attitudes towards her.....and you. You need to sit her down and express your expectations. If you can consider flex time, offer that to her. However, once she changes her schedule, tell her she has to abide by it and cannot change it without consulting with you before.