I had previously used MyFonts.com to buy a web font outright, so I started looking there. Unfortunately, none of the various derivations of Helvetica that contained the name Helvetica were available as web fonts. I did some more digging and found the Helvetica derivation called Swiss 721, which was created by Bitstream. I did a quick visual comparison for several of the letters that appear on the site, and they are virtually identical.
So, if you need to put Helvetica on your site without paying a monthly fee, check out Swiss 721 on MyFonts.com.