This isn't so much an alternative to Helvetica as an improvement upon it, but I mention it here because some of you may like to see it.

Haas Unica: the lost successor to Helvetica, no longer available because of licensing issues, but subtly refined by the same foundry after decades of experience with the original in an attempt to create the ultimate sans serif.

http://www.flickr.com/photos/bauldoff/2389110985/

Univers + Helvetica = Unica