Arial and competitors

First, what is wrong with Arial?  Nothing, really, it is a pretty good font.  Except for the fact that it is not free.  If you have a legal copy of Windows then you have a licensed copy, but other than that you need to license it.  But Microsoft copied Arial from Helvetica, which costs $29 per license. And Helvetica is a copy of Akzidenz-Grotesk, which costs $975 to license.  Tahoma is similar to Arial, and I don't really see much of a difference.  Tahoma also must be licensed.

There are 4 free alternatives.  Arimo comes from Google and is licensed under Apache 2.0.  It is the closest to Arial.  The only differences I see are the upper-case Q and J.

DejaVu Sans Condensed is available under a free license.  There is also a DejuVu Sans, but it is noticeably wider.  Again the upper-case J and Q are different, and some letters are narrower  (see the Ps and Rs).

FreeSans is available from Gnu under the Gnu General Public License. Its Q and J are the same as Arial, but it has an upper-case R with a curved leg instead of a straight leg.  It is a little different in size than Arial.

Finally, Roboto is designed to work with Android phones.  It is available under the Creative Commons license.  The only difference I can see is in the upper case Q.

There are also Liberation Sans, of which Arimo is an updated version of.

Of these, I think I would go with Arimo.  It is pretty much an exact copy of Arial (except for the J and Q).

Finally, how could you go about using it for web content?  Just use the span tag as follows:

[span style="font-family:Arimo,sans-serif;"]text[/span]

Replace the square brackets with angle brackets. So there you go. Arimo should be the default font.