Illum
Flashaholic
I've ahd a very good experience with Pentel P205 drafting mechanical pencils (0.5mm). I actually had one that I used evryday from 5th grade till the middle of 9th grade (someone borrowed it, and it walked away).
Has a good, solid feel in your hand, nice metal tip, good tight clip, has a good eraser, and holds a ton of lead. If you can keep track of it, it'll last you years.
Not overly expensive, but not cheap. My local Staples has a two pack for $7.99.
+1 for Pentel P205, looks professional too... almost "cutting edge"
I have two, one in H and the other in 2B