Arial is somewhat similar to Helvetica -at least in terms of the tracking and line spacing. However, Arial seems more like a mish-mash of other sans styles. Honestly if you put the two fonts next to each other and examine them character by character, Arial is very different.
Bitstream's Swiss 721 is a pretty good knock-off of Helvetica. URW had a pretty accurate knock-off of the Helvetica Neue family, called Nimbus Sans, included with the fonts in Canvas. AGFA's "CG Triumvirate" is another Helvetica Clone.
There are some sans styles quite a bit like Helvetica, but more happening style-wise.
Berthold's Akzidenz Grotesk (billed as "the original sans") is seeing a new burst in popularity. If you have an old copy of Illustrator laying around you may find the complete family in the font bundle -a value worth more than the actual Illustrator application itself. http://www.bertholdtypes.com/bq_library/AkzidenzGrotesk.html