Running a ruby on rails project on my mac. I need to test it on my android phone. Is there a way to view my mac localhost on my android phone?
Chunk's answer is correct, assuming your mobile device and your computer are on the same network. However, if you want your localhost server to be visible to the wider Internet (e.g. for testing over 3G, developing webhooks or collaboration with a remote colleague/client), more is needed as local addresses (starting with 10.* or 192.168.*) are not visible to the wider Internet.
The traditional solution to that is port forwarding and dynamic DNS, but lately a few services have popped up which aim to make this process simpler (disclaimer: I am the author of one of them, PageKite).
These services provide you with a public DNS name and software which connect your "localhost" with an in-the-cloud relay server (a.k.a. a reverse proxy). For example, if you are using PageKite, you can run the following command in the terminal:
$ pagekite.py 80 yourname.pagekite.me
... to create a mapping from http://yourname.pagekite.me/
to the web server running on http://localhost:80
. While the program is running, your localhost site will be visible to the rest of the Internet. In order to make it private again, you simply turn off the pagekite.py connector program.
For completeness, here are some of the localhost tunneling services I am aware of:
(Sorry about not linking to the last two, SO spam protection is preventing me from being fair to my competitors. ;-)