Department member writing reference letter for job applicant for a position in that dept The applicant has asked me for a reference letter for a position in my department. (I can give a positive recommendation.) The applicant is not related to me. Would it be appropriate for me to do so? If another applicant were to ask me for such for the same position after I've sent one for the first applicant, should I decline the 2nd person's request? (That would otherwise be a positive recommendation as well.)
I see no ethical or practical problem as long as you are not otherwise involved in the hiring decision. For your own future, I'd also advise agains any lobbying on the applicant's behalf other than the letter itself. If the applicant is rejected you don't want it to reflect back on you. 

Just be honest, and otherwise keep your distance, just as if you were located in another institution. 

Likewise, giving an honest appraisal of another (or again, another) applicant should be no problem as long as you have some idea of the individual capabilities of each. 

And if you are later asked by your colleagues to rank the various people  you have recommended, avoid it, mentioning that it would be improper and is the job of the committee. 

If I were a member of the hiring committee, I'd have to consider refusing to provide a recommendation, though there might be other solutions, such as recusal.