Is Helvetica a browser base font?
Helvetica is a standard system font on Macs only.
What you can do however is specify Helvetica in a CSS font declaration, but also make sure that you include fallback fonts for Windows users.
e.g.
body {font-family: Helvetica, Arial, sans-serif;}
No, Helvetica is not a "base font".
The best definition of "bast font" is probably Microsoft's Core Fonts for the web, and Helvetica's not there.
If you want Helvetica, you either have to get a Mac (it's one of the default fonts on mac OS X), or buy it from Linotype (the company that owns it).
This is actually a comment to Mark Ransom but it grew to big to be a comment.
Not only is there a a free URW++ clone of Helvetica, financed by GNU as part of the GhostScript project, it is one of the nicest looking Helvetica replacemants out there: URW++ Nimbus Sans. Unfortunately, for a lot of uses, it is a PS Type 1 font.
The FreeSans font (in the openfont file format), also from the GNU project, is based on Nimbus Sans. It comes with most Linux distributions. It is much better looking then Arial (especially in print or on screen when using computer systems that don't support Micro Softs (patented) font aliasing technology) and perhaps even Helvetica, and support lots of Unicode glyphs, but it is not metric compatible with Helvetica, like Arial.
There is a free (in every sense of the word) Arial substitute font, named Liberation Sans, that is metric compatible with Arial and hence Helvetica, but it doesn't really look like Helvetica and not much like Arial either.
Neither Nimbus, Arial, Liberation Sans or FreeSans support all the style variations of Helvetica (and even less all the variations of Helvetica Neue, a font that come bundled with OS X). Nimbus is the most Helvetica compatible of the free fonts mentioned, as it support all the variations that came with Helvetica in a PostScript 2 printer and with older Mac OS. But most modern day wordprocessors (and certainly not web-browsers) can't make use of all the variations, not even those that support PS Type 1 fonts.
Nimbus and FreeSans is under the "GPL with a font exception" license, which allow embedding in documents. Most commercial fonts don't allow embedding without a heavy license fee. Embedding is not allowed for any fonts that come with Windows and OS X, neither for almost all of the fonts bundled with other MS or Apple software, nor do they allow the fonts to be shipped with the digital document to a print shop. With most commercial fonts, the print shop must own a license for the font to print the document, this is especially difficult with Windows fonts, since most (all?) Micro Soft fonts licenses don't allow for use outside MS Windows (the MS Web Core fonts for the Web is no good for print, not metric compatible with newer font versions and support a very limited set of glyphs), so they must also use MS Windows based software everywhere in the preprint chain.