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.